He probably stays logged in at multiple locations(home, work, mobile, etc)
I work from home, so that's only 1 computer. I occasionally log in from a mobile device, but not often.
or has a tab open in his browser always up.
When I'm actively using the site, I rarely have less than 5 tabs open to various sub-forums, discussion threads, or PM messages.
I don't think the "Total time logged in" increases just from leaving a browser window open though. I think, if you don't click any links to send an http request to the web server for a bit, that the "logged in" timer stops until the web server gets another event from your session.