Occasionally, when I check the posts since my last visit, there aren't any, even though there are clearly new posts since the boxes next to the forum names are blue.
That's because this forum runs on SMF 1. SMF 2 keeps track of unread posts that you're following, period. SMF 1 only tracks them since your last session.
It basically shows you all posts between your previous visit and now, and filters the topics that you've already read out.