Difference being that Halo 3 gamers are in majority concentrated in the same region, so Halo 3 peak will be huge. While Steam crowd is quite divided in several regions.
Another thing is that many people don't get online on Steam to play the games. I haven't played online on Steam in weeks, yet I've been playing alot Mount & Blade and a bit of Audiosurf.
And most important of all, just because they are online on Steam and shows not playing, doesn't mean it like that. Many just keep Steam logged on and go play Retail games, or Digital games from other services, and none of those appear on Steam stats.