For example, right now I have about 4 forums with unread posts in them. If I never visit those forums, but come back to this website in a few hours, and if there are no new posts from this point on, then those forums will show up as read even though I never read them in the first place.

It's almost as if it takes a look at the last time you visited the website and only highlights those forums which have new posts since that point, regardless if you had read everything there was to see during your last visit.