If you sign in and immediately close your browser, 15 minutes later, the forum says "he's not doing anything, I guess he's gone... I'll just assume he was here for 15 minutes."
If you sign in, mess around for 5 minute, then sign out, the forum accurately records your time spent online as 5 minutes.
If you sign in, walk away for 10 minutes, then come back and start clicking links for 10 minutes, then sign out, the forum records your time online as 20 minutes
If you do this and instead close your browser rather than signing out, the forum records your time online as 35 minutes.
So for most people, "time online" should be taken with a grain of salt, since I'm guessing most of us don't actually sign out when we leave the forum, we simply navigate to another page or close the browser altogether, in which case your time online is padded by 15 minutes.
My massive amount of online time was garnered by "Click on WNFF tab, refresh, do something else on the computer, click back, refresh"