I honestly think that people did not pick up the game yet. It is foolish to think that everyone who pre-orders will pick it up on release day. People have personal and business lives to deal with, and Gamestop is not open 24/7. Additionally, for a pre-order, you have to go the the exact store you pre-ordered from or your credit is not applied.
Those pre-orders will factor in to the second and third week sales. Don't forget that some pre-order bonuses are given at the time of pre-order, meaning that you can get exclusive content without ever having bought the game.
Shit, I pre-ordered Uncharted 2 and got the free booster. Then, I bought UC2 at a different gamestop. I then finally went back to the gamestop that I actually preordered from and changed my pre-order credit to MW2.
