I mean if you are leaving your computer (for example) but staying logged on you could change your stsus to "away" until you get back. Or if there is no activity detected for more than 30 minutes it could read "not available". SO people wouldn't be peeved if they send you a PM and you don't reply because you've gone to work and left your computer on.
I you leave the computer for more than 5 minutes then the site considers you to be offline. It is based on when you last requested a page, not on whether or not you logged out.