Hi Brett,
That's to do with the way that cookies get set and datetimestamped. It looks at when you were last logged in, and the date and times of posts.
So when you login and logout, you effectively reset it.
If you leave it logged in, it works fine.
I built the recent posts list for this reason - at least then you get the whole lot in date order:-
http://www.voipuser.org/recent_posts.html
Dean