Yeah, Steam sales are crazy awesome, and some of them are bundles that come with games that would cost more if you just bought the one game... so a lot of games I have, I only have because I wanted one of the games in the pack.
Though, looking at my list, it doesn't seem to count time towards DLC and expansions. Though maybe they haven't always kept tracks, since I'm very sure I've played Half Life 2 enough times to have more than just 20 hours on it.
Someone already took my name on PSN so added a - to the end. Probably that punk ass kid who took it on a few forums after seeing me on XBL years back playing Halo 2 I think. I've been using the name since mid 90's so.
I assume that they only started counting time past a certain version of the Steam API, so older play throughs don't show up as apart of that time. Expansions and DLC would show up with the original go, or so I'd hope.