By design, if the arbor is the right length so that it positions the lower lug of the barrel to very lightly press against the frame and at the same time, the arbor bottoms out in the hole in the barrel, it is impossible for the wedge to cause the barrel or arbor to bend making the gun shoot high. By saying, "lightly press against the frame" I'm saying the barrel would have about .001-.005" interference at the lug/frame interface.
In this case, all of the wedge force will be used to pinch the arbor against the bottom of the hole in the barrel.
If the arbor is too short, as the wedge is driven thru the barrel it will move the barrel rearward until the short arbor finally bottoms out in the barrel hole.
By moving the barrel rearward, the lower lug of the barrel will first stop against the pistols frame and as the wedge is forced further in, because the barrel lug won't allow the barrel to move further aft, the arbor will bend, resulting in the barrel pointing upward and the gun will shoot higher than it should.
If the arbor is REALLY short, the barrel will continue to move aft and upward as the wedge is driven further in, driving the cylinder back against the pistols frame at the rear.