Probably because their delivery schedule was full. My wife chose any time for her order, because her only other option was 6-9pm also. Also her order was from Staples and Smart and Final and mine was just from Smart and Final. Maybe if you order from more than one store it takes longer?
Makes sense, although mine was all from Target. Could be something with the scheduling.
Either way - 2 hours or 6 - it's pretty neat.