It does that to me, too, Meadowrose. I think that has to do with remembering cookies on what you've looked at and what you haven't. I erase all my cookies whenever I log out of the Internet, but while I'm on (which is mostly all day), I can see new posts at a glance.
It's not actually a cookie issue; it's a browser history issue. If your browser is set to delete the history when you close it, this would happen.
I, actually, think I have my browser to set to clear both history and cookies upon closure.
