Aya, I think you'll find that there is actually a specific pattern to what you're talking about.
If you click into a forum but don't read all the threads therein, you'll still get a "no unread" when you go back to the top level page.
The server has recorded the time of your last visit to the forum and is indicating that there are no new messages since your last visit. If you go into each forum, you'll still be able to see which threads are unread.
no no, that's why I edited to clear it up. I occasionally get it telling me that the forums are ALL UNREAD... even when I know I've read them... but it still has me own as being logged in (so at least that cookie is working right...)... eh, like I said, a bit of clicking around usually clears it up (but oddly, refreshing the page does NOT fix it up).. who knows *shrug*