Well in the HL2 Discussion forum for example, when I click a thread the person who is listed under "last post" has not actually posted the last post in the thread (nor have they posted the second from last post or the third from last). Or I'm just not seeing the posts after a certain point!
That happens al the time, it's some forum error. I've seen it happen with people like Ians, where it just doesn't show their posts for like a month. It seems to just fix itself or something.
It's just a bug, seen it happen to my own posts. Sometimes you might find certain icons simply not appearing too, pages being slow to load. It's nothing to worry about though, everything still works as it should, just a bit weird sometimes.