The whole reason for official release dates is so that one retailer that gets their shipment in earlier (since this is not a perfect world in which all retail outlets get their shipments instantaneously, simultaneously and magically transported into their store rooms at once) doesn't get to sell their stock before another retailer has theirs on hand, stealing sales from those follow the release dates.
Publishers used to punish retailers for this practice by delaying future shipments of major titles (or at least putting them at the end of the shipping queues).







