I feel like Fallout 3 and Left 4 Dead are probably moving better through retail and Steam. Amazon isn't exactly the marketplace where gamers go to order games so they can wait a few days to play it. Most games I buy are impulse purchases, and if I can't have it now then I can't be bothered to get it. World of Goo seems like the kind of game that parents and non-gamers would buy, and I would guess that that demographic shops more on Amazon.