I don't know if you are aware or not, but it appears that the indicators on the forum software you are using are broken.
After reading a thread, it seems like I have to clear my cookies to get it to show up as read.
*EDIT*
Actually, it seems that if you read the last unread thread in a forum, then go back to the index, the forum will show up as unread until you view the forum itself.
I was wondering about this one - I've had it happen to me before and couldn't figure out how it was incorrect. Caching makes good sense. I'll note it down to take a look. Thanks for letting me know about it.