Are those the comments that also replied to the same comment that you're either looking at or that you've replied to?
My impression (I could be wrong) is that Mastodon treats it like a tree of branching threads, so it will display comments that are on the same fork of conversation that you're on.
So that if you click on the first post, THEN you're looking at the whole tree, and it will show a lot more comments.
@volkris But that's not quite true - I sometimes see other people's comments, not just my own.