That is very strange. As far as the control circuitry for the sander is concerned, the ammeter is a passive device. If you wired it as shown on the reference post, the box containing the meter is just an extension of the outlet on the controller and one of the motor power wires just passes through the meter current coil on its way to the outlet on the box. If you used a meter with a shunt, the results might be different.

Paul