Gamestop does this with certain games i got ACr like this and the game was brand new in a sealed box if u like the game i suggest u take advantage of this asap!
As long as a game is still in print the price is set by the developer or most likely the distributor. The pricing of used games is set by gamestop themselves. It's just a strange quark of business. They probly have way too many new printed copies that don't sell and they need to drop the price of them fast before it's just a total loss.
gs has a habbit of taking new games and opening them to sell at w/e price they want used. just check the top/bottom of a game case to see if it has a razor blade like score across it. if it does was prob opened by gs to sell as used >.>