That generally happens (I think) if the original post count of the thread causes it to go to page x, but posts are then deleted or merged causing the thread to drop back to page x-1. It should resolve itself when it gets back to page x.
I've seen the same exact thing on another vBulletin forum that I frequent. I really only see it in threads that have 250+ pages of 30 posts apiece. I guess those threads have more posts that are deleted or merged because they have more posts in general.