Threads showing up as unread when they shouldn't be

Lately when I check the forums it will show some threads as unread even though I've already read all the recent posts. Example:



I have read all the recent posts from these two threads, I was even the most recent poster in one of them, yet it still shows as unread.

EDIT: and when I checked on this thread, it even said this thread was unread, even though my post was the only one here.

 

I'd actually like to know why this happens occasionally as well...