Why does Hermione say "how often do I have to tell you" when telling Ron that you can't (Dis)apparate inside the Hogwarts grounds?

Question

While writing an answer to Was there any reason to think that there might be students younger than 17 coming for the Triwizard Tournament?, I came across this line:

“A Portkey?” Ron suggested. “Or they could Apparate — maybe you’re allowed to do it under seventeen wherever they come from?”
"You can't Apparate inside the Hogwarts grounds, how often do I have to tell you?" said Hermione impatiently.
_{Harry Potter and the Goblet of Fire, chapter 15: "Beauxbatons and Durmstrang"}

Hermione adds the "how often do I have to tell you" as if she's told him many times before. So, to help substantiate my answer a bit, I decided to look for exactly how many times she's told him that Apparation doesn't work in the Hogwarts grounds. However, I was only able to find one instance where she tells him this, and only one other time it's mentioned before this.

Hermione tells Harry and Ron in The Prisoner of Azkaban:

“Maybe he knows how to Apparate,” said a Ravenclaw a few feet away, “Just appear out of thin air, you know.”
“Disguised himself, probably,” said a Hufflepuff fifth year.
“He could’ve flown in,” suggested Dean Thomas.
“Honestly, am I the only person who’s ever bothered to read Hogwarts, A History?” said Hermione crossly to Harry and Ron.
“Probably,” said Ron. “Why?”
“Because the castle’s protected by more than walls, you know,” said Hermione. “There are all sorts of enchantments on it, to stop people entering by stealth. You can’t just Apparate in here. And I’d like to see the disguise that could fool those Dementors. They’re guarding every single entrance to the grounds. They’d have seen him fly in too. And Filch knows all the secret passages, they’ll have them covered…”
_{Harry Potter and the Prisoner of Azkaban, chapter 9: "Grim Defeat"}

The only other time it's mentioned before this incident in GoF is when Ron is unconscious, later in PoA:

“He must have Disapparated, Severus. We should have left somebody in the room with him. When this gets out —”
“HE DIDN’T DISAPPARATE!” Snape roared, now very close at hand. “YOU CAN’T APPARATE OR DISAPPARATE INSIDE THIS CASTLE! THIS — HAS — SOMETHING — TO — DO — WITH — POTTER!”
“Severus — be reasonable — Harry has been locked up —”
_{Harry Potter and the Prisoner of Azkaban, chapter 22: "Owl Post Again"}

So why does Hermione seem so exasperated here? She's only told him once before. Why the "how often do I have to tell you"?

Answer 1

She only mentions it to Ron once in the text, but given her exasperation at the question it seems very likely that she also mentioned it on occasions which were not shown to the reader. Considering Ron is lazy and shows little interest in learning about Hogwarts itself, it's perfectly understandable that he could have forgotten and needed to be reminded multiple times.

Answer 2

Hermione likely has told Ron several times.

Many conversations that Hermione and Ron have had that don’t involve Harry wouldn’t be seen in the books because Harry wasn’t there to witness them. It’s likely that Hermione has told Ron several other times that weren’t seen by the reader, so therefore aren’t present in the books, since the books follow Harry’s perspective and viewpoint. Ron and Hermione are together without Harry often enough for there to be several of their conversations not in the book. For example, they’re both in the stands together at Harry’s second-year Quidditch practice.

“As Harry walked onto the pitch, he saw Ron and Hermione sitting in the stands.

‘Aren’t you finished yet?’ called Ron incredulously.

‘Haven’t even started,’ said Harry, looking jealously at the toast and marmalade Ron and Hermione had brought out of the Great Hall.”
_{^{- Harry Potter and the Chamber of Secrets, Chapter 7 (Mudbloods and Murmurs)}}

Though the Harry Potter books are told in the third person, they mainly follow events in Harry’s life, and don’t tend to follow anyone else, of course the other people would still exist in-universe when not currently featured on the page. Very many conversations between other people happen without Harry’s knowledge - Draco tells Crabbe and Goyle (though really Harry and Ron using Polyjuice Potion) that his father Lucius always says Dumbledore’s the worst thing to happen to Hogwarts.

“I suppose Dumbledore’s trying to hush it all up. He’ll be sacked if it doesn’t stop soon. Father’s always said Dumbledore’s the worst thing that’s ever happened to this place.”
_{^{- Harry Potter and the Chamber of Secrets, Chapter 12 (Polyjuice Potion)}}

These conversations between Draco and Lucius would still have happened, but they aren’t seen in the book because they were had when Harry Potter was not present. For it to actually be provable using only the text of the books that Hermione has only told Ron that Apparition won’t work in Hogwarts once, Harry Potter would have to be omniscient, hearing every conversation that everyone has over the course of the series.

Answer 3

There are several other times in the series where we see Ron and Harry make the same mistake again and again. When Harry somehow got out of Hogwarts and turned up in Hogsmeade in Chapter Ten of Prisoner of Azkaban we find:

"Wow!" said Ron, looking very impressed, "you've learned to Apparate!"

"'Course I haven't," said Harry. He dropped his voice so that none of the sixth years could hear him and told them all about the Marauder's Map.

In Chapter Twenty-Nine of Goblet of Fire when discussing how Mr. Crouch had disappeared, we find:

"I don't think so," said Harry, shaking his head. "He seemed really weak – I don't reckon he was up to Disapparating or anything."

"You can't Disapparate on the Hogwarts grounds, haven't I told you enough times?" said Hermione.

Mere hours later when discussing the case with Moody, we find Ron making the mistake again:

"So he did Disapparate?" said Ron.

"You can't Disapparate on the grounds, Ron!" said Hermione.

In Chapter Twenty-Three of Order of the Phoenix we find Harry making the mistake again:

“This was different,” said Harry, shaking his head. “I was inside that snake. It was like I was the snake... What if Voldemort somehow transported me to London — ?”

“One day,” said Hermione, sounding thoroughly exasperated, “you’ll read Hogwarts, A History, and perhaps that will remind you that you can’t Apparate or Disapparate inside Hogwarts. Even Voldemort couldn’t just make you fly out of your dormitory, Harry.”

By the beginning of Half-Blood Prince this idea seems to have finally penetrated Harry's head:

"— you can't Apparate anywhere inside the buildings or grounds," said Harry quickly. "Hermione Granger told me."

This makes at least six times that Harry and Ron mentioned apparating in Hogwarts (or the grounds). If it happened this many times during the parts of the story we are privy to, it is likely that it happened other times as well. For all we know, Harry and Ron were making apparition suggestions on a weekly basis. (If it were me I'd probably do it deliberately just to annoy Hermione.)

It's also possible that a perfectionist genius like Hermione would consider one time to be more than enough. Thus, even if the case in question truly was only the second time she told this to Ron, in her mind that could be grounds for the reprimand. Alternatively, Hermione may not have been keeping an exact count of Ron's gaffes. She would have been correct in remembering that she had informed of this before, and she would also have been correct in remembering that Ron had already made the same mistake again on an earlier occasion. In her mind that might have translated into the notion that she had told him multiple times.

A final option is that the author is the one that made the mistake. She clearly intended there to be a recurring problem that Harry and Ron couldn't process (or remember) the fact that apparition is blocked at Hogwarts. She may have simply lost track of how many times it had already occurred at that point, especially if she didn't write the book straight through from beginning to end.