Bundles are a big contributing factor to this. I often purchase bundles because I want to play 2-3 games in them, and they're cheap enough that it's very often worth buying a bundle for just two games in it. There are a number of games that have been in bundles that had yet to break the bundle price even individually.
Then, when you get the bundle, you can either redeem the codes for the games that you're unlikely to play, or let them sit.
Though when I buy games individually, by now it's almost always when I have time to play that particular game... though I still haven't found time to play Tomb Raider, which I bought over Christmas for $5
Though just for fits and shiggles, I checked my Steam account. Yes, I do own Ricochet. I have no idea when I got it, but I assume that it was a fire-and-forget bundle redemption.