That's fairly common, in fact Winchester 1300's are advertised as the "speed" pump as a result. The action unlocks at detonation and if your hand is on the pump, the recoil will allow your hand to move the handle back. The shot charge will already be out of the muzzle when that happens. You will notice it more on a heavy load than a light one. As long as you can't shuck the action when it's cocked, there's nothing wrong with the gun.