You won't ever get 0 RPM while in flight. The air pushes the prop and keeps it spinning. You are "set to 0 RPM" but the game just gets you as close as you can.
Same goes for setting high RPM. You might "set to 100% RPM" but depending on your altitude and power curve, you could develop a lot less.
EDIT: What is reversed? The throttle, or the RPM? Make sure you are reversing the axis for the right one!