Keyboard shortcuts and motor temperature

I have a step-down converter with a current and voltage display. I have connected this to power the motors only. At 5V with all three motors energised it is 0.57A, so about 2.8W for the three.
Interestingly when x or y motors move it drops to 0.55A, but when z moves it drops all the way to 042A. I shall clearly need to check whether this is a difference in the motor or a difference in the stiffness of the z-axis.