Calculation of supply current in stepping motor driven using chopping driver
Yes, you are right, this is 'something to do with the chopping'.

Look up how a Buck DC to DC step down converter works. The inductance of the stepper motor acts as the inductor in a buck converter, while its resistance acts as the load resistance.

With the driver acting as a loss-less step-down buck converter, the power supply only needs to supply enough power to meet the power losses in the load. As the voltage is high, the current can be low.