Why was Hagrid expecting Harry to know of Hogwarts and his magical heritage?

Question

It is obvious that someone (at least Dumbledore) knew about the conditions Harry's been kept in: the letters were addressed to "cupboard under the stairs" and then to "the smallest bedroom". Moreover, they knew Harry wasn't receiving his letters, hence the hundreds of letters, sneaking them in the milk bottles and flowing through the fireplace.

Given that Dumbledore trusts Hagrid enough to get the Philosophers Stone from the Grinngotts, and prior to that to deliver Harry to Godric's Hollow, and overall his trust in Hadrid is doubtless:

Professor McGonagall opened her mouth, changed her mind, swallowed, and then said, "Yes -- yes, you're right, of course. But how is the boy getting here, Dumbledore?" She eyed his cloak suddenly as though she thought he might be hiding Harry underneath it.

"Hagrid's bringing him."

"You think it -- wise -- to trust Hagrid with something as important as this?"

"I would trust Hagrid with my life," said Dumbledore.

_{Harry Potter and the Philosopher's Stone - Chapter 1 The Boy Who Lived}

So it's rather unlikely that Hagrid was unaware of Harry's knowledge or condition, because why would Dumbledore keep something like this from Hagrid while fully trusting him?

The out-of-universe answer suggests that

Essentially it provided JK a device to reveal some of the key information to the audience, as they were surprised with Harry.

_{Reddit answer here}

I've found these questions on reddit and quora, but most of the answers suck and none of them give an in-universe explanation of this.

Answer 1

Hagrid never met Petunia or Vernon before, and never saw them in between dropping Harry off after his parents died and bringing him his letter on his 11th birthday. So he has no idea what they are like, or how they treated Harry.

However, he knows that Petunia is Lily's sister, and would know about the existence of the magic world, as the immediate families of muggleborns are informed. He knows Lily went home for school holidays and such, so probably assumes that she told Petunia more as well.

Also, magical kids are known to show instances of accidental magic.

So Hagrid just assumes that Petunia would have told Harry about his parents and about magic, as one would expect, since she knows and is the person raising him, especially once he started showing accidental magic.

Answer 2

Dumbledore understood that Harry was not getting his letters and probably is mistreated and might not know about Hogwarts. He therefore told Hagrid:

"I never expected this," he said, in a low, worried voice. "I had no idea, when Dumbledore told me there might be trouble gettin' hold of yeh, how much yeh didn't know.

Dumbledore himself might've not known how much Harry didn't know- He only knew that he was not getting his letters and probably being mistreated, so he just told Hagrid in general that there might be trouble.

Also: Dumbledore told Hagrid that "There might be trouble"- That is a little vague, but it wasn't really necessary for Hagrid to know about Harry's level of knowledge- The main mission was to get Harry out of there.

Dumbledore also might've told Hagrid everything, but Hagrid wasn't paying attention or misunderstood Dumbledore. Hagrid saw Dumbledore putting the letter with Harry when they delivered him to the Dursleys. He probably subconsciously had in his head that Harry obviously knows about magic.

Either way you look at it, The fact that Hagrid didn't know how bad it was, isn't because Dumbledore didn't trust Hagrid, but because of a misunderstanding from either side.

Answer 3

It is probably a combination of reasons.

• Hagrid is not the brightest person, and not the right person for the job, so he doesn't figure it out himself.

  “Ah, Harry, I don’ know if I’m the right person ter tell yeh.” (PS)

  Even assuming Petunia was willing to tell Harry what she knows, how is she supposed to know?

  “But yeh must know about yer mom and dad,” he said. “I mean, they’re famous. You’re famous.”

  How would Petunia know that?
  Was she supposed to tell Harry that yes, magic exists, no, she doesn't anything about it?

  Hagrid knew he had the letter to deliver, that wouldn't make sense if Harry already got the previous letters. He knew the Dursleys were on a lonely island somewhere, but owls can deliver there, as seen with the prophet. So he has the information to deduce that the Dursleys keep the letter from Harry, but he doesn't figure it out.

• Dumbledore on the other hand knows how Harry is treated, in fact he knew before he placed Harry there:

  “I knew I was condemning you to ten dark and diffi­cult years.” (OotP)

  Dumbledore also had Mrs. Figg watch Harry, and she said:

  “I’m sorry I gave you such a miserable time, but the Dursleys would never have let you come if they’d thought you enjoyed it.” (OotP)

  So she clearly knows about the Dursleys' attitude towards Harry, and she reports to Dumbledore, so he knows, too.

• But Dumbledore didn't tell Hagrid

  “I had no idea, when Dumbledore told me there might be trouble gettin’ hold of yeh, how much yeh didn’t know.” (PS)

Now we don't know how the Hogwarts letters are created and sent, so there is a sight possibility Harry's letter was signed by McGonagall, but she didn't know that it was addressed to Harry's cupboard. But it doesn't make sense to assume that the following letters were also sent automatically without anyone knowing about them.

As Harry is not just any student, it is certain that Dumbledore knew that Harry didn't get the letter, but figured that sending Hagrid without that information was best for Dumbledore's plans.

Answer 4

Hagrid was a close friend of James and Lily. Of course, Harry would be dear to him too. He also rescues him from the ruins of the Potter’s house in Godrick’s Hollow.

This might have made him feel somewhat protective towards Harry.

Harry was constantly mistreated and used at the Dursley’s. If Hagrid (on a slightly different note, anyone on the wizarding world who is not a Death Eater; with some exceptions of course) knew about how he was being treated, he would have tried his hardest to make Dumbledore change his home. He would have wanted to keep him or at least would have vouched to relocate him.

But as Dumbledore knew it was very important for Harry to stay at the Dursley’s for a few years, he might have withheld the information of how Harry was, and how he was being kept.

So from Hagrid’s perspective, the little boy whom he saved is being cared for nicely at his aunt’s house. Being constantly reminded of how brave his parents were, and how he defeated the Dark Lord and how respected his family was among the wizarding community

When Hagrid is told to give Harry his letter personally, that’s when Dumbledore mentions there might be trouble for the first time. This is why it came as a surprise to Hagrid.