Yes, I've noticed this, and there's rarely a good reason for it.
Occasionally writers do this deliberately to spring a surprise on the reader. Like, "ha ha, all along I had you thinking these were people but really they're mice!!" If done well this can be a clever plot twist. If done poorly the reader just feels like the whole book up to this point was a fraud.
I recall that in Isaac Asimov's "Second Foundation", there's a point -- and I'll avoid giving details to avoid spoilers -- where he suddenly reveals that two characters who the reader has almost surely assumed to be different people all along are really the same person. (This would be awfully tough to do in a movie: wouldn't we see the character's face and realize it's the same person? It would be suspicious if one or both was always seen only as the back of his head, or always wore a mask, or was always in shadows. But in a book it works.) It works because it suddenly brings things together, the reader's likely response is, "Oh, now I see why ..." and "now I get it". But I can easily imagine a writer doing this poorly and the reader saying, "What? That was a cheap trick to solve the problem!" or even "Huh? What do these two have to do with each other? How could they be the same person? Wasn't one of them in place X while the other was in place Y?" Etc.
Usually, though, I think it's just that the author got sloppy. He pictured in his mind that George was a very young, strong and muscular man, forgets to mention this to the reader, many readers picture George as a thin and frail old man, and then suddenly there's a scene where George saves the day by lifting the car off the accident victim, he mentions at that point how strong George is, and now the reader has to re-visualize every scene.
The lesson authors should learn is: If there's something about a character that is important to the story, and that would be obvious when anyone first sees him, be sure you mention it when he is first introduced.
Of course this can create a problem with getting a story or a scene started. You don't want to beat descriptions to death. The reader doesn't necessarily need to know that a character is wearing a ring on the third finger of his right hand, that his shoelaces are black, or that he has a small mole behind his right ear. But tell the reader the things that would be obvious about the character to anyone seeing him when the reader first sees him, and that matters to the story. You can often do this very briefly. Like, "Tom was well-dressed" may be plenty of information. You don't have to specify the exact color of his tie and the brand of his cuff links. But if a scene begins, "Tom was working on his farm", then unless you specify otherwise, I'm going to assume he's wearing blue jeans or overalls. Don't tell me five pages later that he's wearing a tuxedo.