Just out of curiosity how did you get your flowrates? Unless you took return line off and actualy measured the flow returning or have a small accurate inline flow meter I would bet you only flowing 3.5 LPM (or less) at the pumps designed head pressure at both voltages.

These are non positive discharge pumps. You could have the designed head with zero flow once it reaches it's designed head pressure, then the water just sit's there and spins (slippage) and builds up heat energy. I once saw a pump hooked up to PVC build up enough heat in the water to soften the PVC to the point where it swelled up and burst because someone turned off #1 pump and shut #2's discharge valve when switching pumps. It was a lot larger pump granted but still the same theory.

If you were to put a ampmeter, a flowmeter, a pressure gauge and a thermometer on a pump then slowly shut down on a discharge valve the pressure would rise to x with y flow and z amps. Keep shutting down on the valve and x and z would reach a point where they remained constant but y would continue to decrease until it hit zero. Study Newton's law of energy conservation look at the thermometer in that discharge line and you will see where the energy( x timesz) went.