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.
The "new forum" thing is a minor glitch I have been seeing for years, but I've never noticed it causing any real problems with the read/unread. (Maybe I just didn't notice.)