Perhaps you can come up with an alternative answer as to why some have experienced higher RoD in auto without the engine running compared to with it at idle.
Well, I did make suggestion in a previous post. But it may depend on what else is still rotating such as the cooling fan or other engine driven attachments rather than driven from the gearbox. I do not know, but would be inclined to think that a turbine may also show a diffence for the same reason, especially a H500 with the engine at an angle.
Better ask Nick Lappos!
Also the orientation of the cylinders in a piston engine. It is too long ago to recall just what the H300 did, but I have more time in an old Hiller, and I just do not remember if there was any difference. The point being here the difference in the engine layout between the two examples. I do know that it was quite easy to stretch the glide in the Hiller, but then the rotor system was more akin to a Huey.
The first R22 that I owned was before the tip weighted blades were introduced, and that required immediate and judicious use of the collective to the extent that it would not have been noticeable either way! Never did one with the engine stopped, just at idle.