Two reasons.
First of all, when I replied to this thread earlier my replies should have made a new page because the page before had 15 posts. Makes sense- they were replies 450 onwards after all. Now, instead, those replies are on the page before.
Second, subtract the amount of posts on this page from the amount in total and you get 452, hence this page starts at 452, and hence it is not a multiple of 15, clearly enough. The extras being from the ones I made that didn't show before, I guess- though what page now has 17 posts, I do not know.