in 2+ engine planes in AH, when you shut the engine down, you also auto feather the prop. This reduces drag dramatically, vs. throttling back at high RPM that keep the props windmilling and produce huge drag.
Try to land a Mosquito or P38 in a glide with engines off and with throttle back (full RPM).
In single engine planes there is no prop feathering, so to the best of my understanding, there should not be a difference between full throttle back and engine cut.
If someone wants to test the drag difference, put the plane in auto-speed, throttle back / cut engine and note the rate of decent.
For Pipz,
my perception of the RPM changes in the Mosquito is that I cannot reduce RPM below a certain value, even when the speed is low and the engine stays laud in a throttled back glide. This confused my in the beggining - I kept thinking that my throttle is broken or uncalibrated and does not reduced power.