Now I care - accidentally closed my browser. Opened up browser and this time the list of unread posts was only a few in comparison with three pages previously. I guess it is right though, in that these are unread since my last visit. I generally never close the browser so never have any problems with the unread posts (never turn the computer off, either, unless I have to). Maybe this will help with consideration of this issue.
I would prefer to see all unread posts, rather than just those unread since last visit.