Quite. - it may be that the Master is actually trying to move more air, hence is working harder, hence the lower RPM ?

I often wanted to mess with a fish weighing scale, attached to the rear of a wheeled (dolly if ness) model, on a flat surface, might give comparitive type thrust measurements ?

My ST G20/15D performed at it's best in the air with a wide blade Master Airscrew 7X6.
The later M.A. 7X6 gave more revs but less airspeed, (more like using a 7X6 APC)