Try it at different power settings, see if it's related to the slipstream somehow? If that doesn't make much difference, then maybe the airplane is not trimmed in yaw? Even though the rudder has the same throw in both directions, the plane could be flying with a yaw angle. Since we don't sit in the cockpit with a turn and bank indicator, there's no way to know if it's off a little bit? Maybe try using the centerpoint adjustment to offset the rudder to the right, retrim the ailerons and repeat the experiement? Or maybe the engine thrust line might have the same effect.