Yeah, I'm not sure what that stat is showing really - I was in the top ten for "most time online" when we switched to the new forum and it's stats, and I hadn't been a member that long at that point... and I'm sure the number goes up and down sometimes?
I think it's more to do with the fact that I never log out on the three different PCs I might use, I just set "Stay logged on" to save me ever having to log in.