Did the behavior of the Summoning Charm change between Goblet of Fire and Half-Blood Prince?

Question

Goblet of Fire, mostly through Hermione, repeatedly tells us that the Summoning Charm requires concentration. The following lines in chapter 20 further imply that you need to concentrate on the object that you desire to summon:

Just as long as you’re concentrating really, really hard on it [the Firebolt], it’ll come.

It was time to do what he had to do... to focus his mind, entirely and absolutely, upon the thing that was his only chance. ...
He raised his wand.
"Accio Firebolt!" he shouted.

However, in chapter 27 of Half-Blood Prince Harry successfully summons two objects that he's presumably never seen before ("Accio Rosmerta’s Brooms!") and in chapter 26, the cave reacts to Harry attempting to summon an object that, unknown to him, he's definitely never seen before.

I'm referring to "Accio Horcrux!". Recall from the later chapters that the fake Horcrux does not look identical to the real one. Moreover, he didn't know ahead of time that the locket was the horcrux in question. For all he knew, any of the as yet unknown horocruxes could've been in the cave, so how could he have known what to concentrate on?

In the second case, I can handwave the cave's reaction as it simply reacting to an attempt to use the Charm, but Harry should've realised that it was impossible to attempt the Charm in the first place. As for my other example, what Harry was thinking of is even less clear. As Harry no doubt knows, not all brooms look the same.

Is this an inconsistency in the behavior of the Summoning Charm? As of Half-Blood Prince, Harry appears to be able to summon objects that he can't picture. It gets even worse in chapter six of Deathly Hallows, where Hermione summons some books that she's never seen and doesn't even know the amount of.

Answer 1

No, Because it likely works based on an object's innate qualities

The first appearance of the summoning charm is in Harry Potter and the Goblet of Fire when Mrs. Weasley uses Accio to discover/confiscate all the Ton-Tongue toffee Fred and George are trying to sneak out of The Burrow. She certainly didn't know where all the toffee's were apart from "Somewhere maybe on the twins" and may not have seen the actual toffees or the specific candies. Yet the charm works. I would say that Hermione's indications that the charm is very difficult is more "difficult for Harry/untrained wizards" rather than "A difficult spell in general." Her admonition to "concentrate very hard" on the specific object is probably right, as far as it goes. But it may also be her wanting to be as sure as possible that Harry will get the spell right.

Furthermore, accio in general seems to be a summoning spell that works as long as you have a clear idea of the object in question, but not necessarily the physical aspects of said object. For example, when Harry summons Rosmerta's brooms. He's likely never seen them before and is likely not to know their exact location. so he's not concentrating on "the Cleansweep 7s in the upper left room of the pub one of which has a chip out of the handle and one with a tail-twig slightly out of alignment." He's thinking "the bewitched flying implements whose owner is Rosmerta, this person I know."

Likewise Harry's attempt to summon the Horcrux wasn't forgone to failure (accio the-thing-precious-to-Tom-Riddle-on-that-island-over-there) it failed because of defensive enchantments.

Or think of this Deathly Hallows scene:

Within seconds Harry heard a weary, male voice.
‘There ought to be a few salmon in here, or d’you reckon it’s too early in the season? Accio salmon!’ There were several distinct splashes and then the slapping sounds of fish against flesh

Obviously the summoner can't have seen those specific fish, but summoning based on "I have an image of what a salmon is intrinsically in my mind and I want the ones/one near me over THERE" worked. Just like for Mrs. Weasley's toffee summoning.

I will say that this sort of "lateral thinking" about summoning a thing based on its intrinsic features rather than its outward appearance might be hard to wrap ones head around, so Hermione's statements of how difficult it will be for Harry to do it might be related to the fact that concentrating on every little detail of an object (Harry's firebolt) might be easier to wrap one's head around, but need more focus to accomplish.

TL/DR: The spell's attributes don't change between Book 4 and Book 6, only the skill of the caster.