@Ladynoosy - That usually happens (sometimes) when posts are deleted in a thread. The server counts the deleted posts as it's able to see them, but since we can't view them they're not accessible to us (normal members).
I think the only way to fix this for now, would be to hard-delete the posts that were soft-deleted. In short, there is nothing on the last page for us to view.. the deleted posts might be on that last page, or somewhere random in the thread.
It's pretty much a vBulletin server error. Posting in the thread sometimes fixes it, but it'll keep coming back, especially with larger threads.