Maybe this has to do with the upcoming Xbox 360 launch.  A lot of people trade in their old games and systems for money to buy the new consoles.  Perhaps Gamestop feels that if they payout a little more then they can sell more Xbox 360s and thus more games, more accessories, etc.  It's in a game store's best interest that people own the newest consoles because that's where most of the money is going to come from.