Just thinking out loud. A higher ROD would imply more energy is required to maintain RRPM with the engine out vs at idle. At the idle detent you're getting 50-60% Ng which is providing a fair amount of drive to overcome some of the inherent drag and friction in the rotor system/drive train. With no engine you would need to overcome 100% of the forces via autorotation, which would require a higher ROD.