If a sentence would say:
X has nearly as many Ys as Z.
I think it would be understood that it means that X has fewer Ys than Z has, but it is not that much fewer. I.e. X has almost as many as Z. When you put a negative in front of the sentence the meaning changes. The question is how much.
If we instead write:
X has not nearly as many Ys as Z.
I would think that this means that X does not have almost as many as Z (because the previous version meant that it did have almost as many, and all I did was add a “not” to the beginning). If a sentence states that X does not have almost as many as Z, I would think that it implies that X has somewhere near almost as many as Z. I use “somewhere near” in contradistinction to “nowhere near”. That is to say, that “not nearly as many” does not mean the same thing as “nowhere near as many”. The latter implies that the amounts of X and Z are extremely far apart, while the former implies that there is only a noticeable gap between them but they are not so extremely far apart.
However, when I googled the phrase, I found the Collins English Dictionary which says that in American “not nearly” means:
not at all; far from
And in British it means:
nowhere near; not at all
This seems strange and counter-intuitive to me. Is this correct, or was my above assessment correct? (Or some other option?)
The relevance of this question is whether I can make an inference from the following text:
Where was this place? It surely wasn’t Hogwarts; he had never seen a
room like that here in the castle. Moreover, the crowd in the
mysterious room at the bottom of the basin was comprised of adults,
and Harry knew there were not nearly that many teachers at Hogwarts.
(Harry Potter and the Goblet of Fire Chapter 30)
The text later tells us that the actual number of people in the room was at least 200. Can I infer from this that there must have been many teachers at Hogwarts (say, 50-100 as opposed to only a handful) because otherwise it should have said “nowhere near that many teachers” and not “not nearly that many teachers”, or would both phrase choices actually mean the same thing?
(Note, I am not asking whether a particular author was aware of any differences in meaning, or was precise in word choice. I am asking whether objectively speaking the phrase “not nearly” carries a certain meaning/implication.)
In this case, not nearly means just that. The suggestion you make (that it should otherwise be nowhere near) is wrong only because the latter is a more emphatic way of saying it.
To put it another way, not nearly is a literally understated way of expressing nowhere near.