It's all about the "session". A session consists of when you log in and are "active" on the forum. If you get up to go grab a refreshing beverage and get stopped on your way back to the PC, your session can "time out". So, when you get back on the PC, a new session begins. You click "New Posts" and there are zero. Well, that's because there are zero posts since your last session.
That's how I understand it and that's about the only way I've seen on my PC what you are experiencing from time to time. My kids have a way of pulling me away from this @!#[email protected]
I hope that helps. Thanks for being a part of ROC!