That's not actually a bug.
It's usually only noticed on longer, multi-page threads. What happens is that the forum software is set to display 35 (or whatever it is) posts per page of a thread. Sometimes, though, a moderator has to delete posts from a thread. When that happens, the posts stay in the database and are counted as part of the page count, they just aren't displayed to anyone except moderators, and it just shortens the number of pages for everyone else.
For example, if we have a thread that has 70 posts, and is set up for 35 posts per page, it will be 2 pages long. Then one more person posts, and it goes to 3 pages. However, someone has broken the rules, and a moderator deletes one of the posts. The post is still there, but it just doesn't display, so normal users (like you) will only see 2 pages of posts again (the 70 remaining posts at 35 posts per page), even though it will say there are 3 pages of posts.
This gets exaggerated in threads that are tens or hundreds of pages long, obviously.
There is a setting to make it re-paginate everything, but we haven't turned that on, because our forum database is so large that it would cause a LOT of forum downtime to re-process everything.