Well, it isn't unheard of for individual Gamestop stores to break the ship date on games. I've no clue if they've got their stock yet but that seems the most likely cause.
It's actually the street date, not ship date. There's a bit of difference between the two (primarily that a store is allowed to SHIP orders prior to the release date so they arrive at their destination on the release date - that's called a ship date - a street date is the date that stores are allowed to put the game on shelves and sell it to people coming in off the street, the game's developer-set release date).
Well, they probably did ship some units early. I know some people who ordered online and they have been charged for the item even though there's a clause that says gamestop will not charge until the unit ships.