There is some sort of strange bug, which is most likely associated with cache/update process. The problem is that unread count doesn't drop on index page, until all post in given board have been read. For example, if I have 3 unread posts in 3 topics in 1 board, and read only 1 post, counter will still display 3 unread messages on index page. I don't know if this is how it is supposed to be, and that's why I am calling it bug. But I must say that if that's how it is supposed to be, then that modification is quite useless, because knowing wrong number is worse than not knowing at all.