The most likely explanation I have is that the "unread topics since your last visit" does exactly that it shows only the unread posts that have been made since your last visit. If you visited the forum, forgot to mark any threads as read, then leave and come again, the unread threads from your first visit won't show up again, unless you click on the "show all unread topics" link.
This makes sense in your screenshot as the Podcastle forum was the only one with unread posts from the previous day.
Let me know if you think this isn't the case.