I believe it's random based on when people post. The New Posts brings up all the posts made since the last time you read them. Sometimes there are fewer members on line.
What I do is read all my New Posts (sometimes 50-80 posts since yesterday) then go back and click on New posts a second time to pick up any arrivals while I was reading the first batch. When I'm sure I've seen them all, I Mark Forums Read so these posts won't come up again.
I hope that helped rather than confused...
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan