ahha!!

In Part A, the string has a mass itself but we didn’t measure it, the washers have slightly varied masses, the circles formed when we rotate the instruments are slightly slanted. Movement of hand increases/decreases the speed of the rubber stopper.
In Part B, the string moves around inside the tube, which creates the different radii, and the circles formed when we rotate the instruments are slightly slanted. Also during the rotation, the base (washers) doesn’t stay idle, it moves around, which makes the Fg different....