Maybe it's just my setup but most posts are marked as read when I exit the site and log back in. It's almost like the system is resetting the dates on the access.
For example most of the unread posts listed right now will be flagged as read the next time I log in.
I've noticed the same thing but I don't find it a huge problem. Occasionally it is a mercy in that it causes you to overlook posts you would never have wanted to reply to.