I've noticed that sometimes just clicking on "new posts since last visit" doesn't give all posts that you have not read (if you see what I mean). If I keep the bpl site open but don't use it for awhile, then click new posts, it can sometimes not show several posts that will be seen if you click "last 50 posts".
So, if you leave a window open for awhile and then click new posts, you could very well not have any new, but have several that you have not read. Does this make sense?
I'm on a G4 powerbook running OS 10.3.9 and firefox.