The wear pattern on the replacement pump may just mean the arm isn't well constructed. That could happen in today's quality control environment.
You could see if the cam lobe (an off center circle like buoymaker said) is worn by comparing the angle of a screwdriver held against the top of the cam and the top of the pump opening in a couple of different positions of the cam. But I have my doubts it would be worn enough to cause the pump to quit working in a short period, and there would likely be a LOT more wear on the arm of the original pump.
