Not to bang too hard on the same drum, but 4e is not a game in which the numbers "represent" or "reflect" any fiction. They are an action resolutoin device.
I'll requote from
Vincent Baker to emphasise the point:
Roleplaying is negotiated imagination. In order for any thing to be true in game, all the participants in the game (players and GMs, if you've even got such things) have to understand and assent to it. When you're roleplaying, what you're doing is a) suggesting things that might be true in the game and then b) negotiating with the other participants to determine whether they're actually true or not. . . .
Mechanics might model the stuff of the game world, that's another topic, but they don't exist to do so. They exist to ease and constrain real-world social negotiation between the players at the table. That's their sole and crucial function.
What Mr. Baker fails to note in that quote (though for all I know addresses it elsewhere) is that the numbers serve another purpose: they provide the framework via which the players (and GM) can quantify elements of the fiction that need quantifying in order to give a playable game: base stats, combat skill levels, toughness, armour, etc. Put another way, you say the numbers in 4e aren't a model of the fiction, but if they aren't then how are the players (and the GM, for that matter) expected to mechanically interact with the fiction on any sort of internally-consistent basis - what model can they use, if not the numbers?
The numbers aren't always absolutes in and of themselves, but what they do accomplish is to define relative differences between one thing and the next...and (here's the key bit) the next and the next and the next,
all at once.
This is the key bit why, you ask? Because, using good ol' Joe the bugbear from the last post, Joe's 35 hit points tell us his relative toughness in comparison to every other creature in the game world...including but by no means limited to whatever he's fighting at the moment. Joe's 7 intelligence gives us a good idea of how smart he is not only in relation to other bugbears but in relation to the world at large. His current AC of 15 (or 5 if using descending AC)* tells us how difficult it'll be to get through his amrour and hurt him, no matter who or what is attacking.
* - of these three examples, this one lines up best with narration and imagination: it's pretty easy just on simple first glance to narrate armour so as to give everyone a reasonably good idea how well-defended a creature might be...which means by extension if a creature's AC changes later, there has to be a corresponding change in the fiction to account for this - better armour, or some (maybe lootable!) magical protection, or something. Changing the AC without suitably changing the fiction makes a mockery of it all - it's like a codified version of a GM pulling numbers out of thin air just because she can.
Now, how does this relate to locking in the numbers? Simple. Joe might be a pushover when faced with a 17th level warrior but his toughness relative to the rest of the world hasn't changed; and the GM can't just change that on a whim without a good in-fiction rationale.
But that's only part of the equation here...
4e's combat mechanics - hp, to hit numbers, damage, defences - are an action resolution framework. They are not a model of the fiction.
The numbers in 4e are a resolution system. When the resolution is different - eg the bugbear is facing PCs who are far more powerful than it - a resolution system is adopted that gives expression to this. All the numbers - defences, to hit, damage and hp - are changed. (You seem to be focusing only on the hp - I don't know why.)
4e's mechanics have nothing to do with "consistently interacting with the rest of the ficitonal world". That is not action resolution. It is not establishig the shared fiction by way of negotiation. Given that 4e is a relatively traditional RPG in its allocations of authority, the GM just makes that stuff up.
Bugbears are made of flesh, not sand.
And as has been mentioned a few times in this thread already, 4e is ficiton first. The players' know the fiction in the same way that they know the fiction of a film or novel - from imagination and description.
And the other part of the equation is this: all too often imagination and description simply aren't good enough, even if the GM gives the most in-depth narration you can conceive. Why's that? Because narrating the same scene to four different people is almost certainly going to paint four different pictures, one each in the imagination of each listener. And when you add that none of those pictures might match the actual picture the narrator is trying to describe, unless the narrator has a drawing or photo of the scene to bring everyone together you're inevitably going to get questions and misunderstandings; which IME can lead to some thunderous arguments if players base their actions on imagined or mis-interpreted scene elements that differ from what the narrator had in mind.
Numbers can help with this, and maps, and all those other so-called fiddly things. Still probably not perfect, but better than before.
And that's just a scene. Now think about trying to narrate things that can't so easily be pictured, such as (on first observation) a creature's toughness or intelligence or combat skill relative to the rest of the world. Here's where numbers become essential, not so much to help the narration but to guide the GM (usually, but sometimes the players) in playing and-or interacting with that creature and in what makes it tick, just like a player uses the numbers on a character's sheet as a mechanical representation of what that character's all about.
And for this mechanical representation of the fiction to work in any sort of consistent and trustworthy manner, the numbers, once set, have to remain so unless something materially changes about the creature**. The players have to be able to trust that the setting is internally consistent enough in its mechanics that the ogre they met (and fled from!) at 1st level is mechanically going to be the same when they meet it again at 15th level, or when they go back to town and send their bosses out after it.
** - just like a PC.
Re the bolded bit in the quote above: I focus on h.p. as it's the most egregious and obvious change to a creature's stats caused by minionizing it.