Bally Heck,
If the engine is at idle, as Multpb stated, then the engine is self sustaining and AVGAS provides the energy to drive the engine and its ancillaries. As the prop is being driven round it will still provide some thrust and therefore an idling engine will give a reduced ROD over a stopped prop.
However, with a failed engine and a windmilling prop, the energy to drive the engine round comes from the potential energy of the aircraft i.e. its height. Airflow alone is driving round the engine, pumping its oil, driving its electrical generator, overcoming friction, compressing air etc. This will cause an increase in ROD over a stationary prop.
ShyT
