3D,
I didn't ignore your original question, I just can't answer it. I do note that the data on Dr. Leishman's site shows no such forward speed bias, but his data is rotor only.
I have a gut feel as to the answer. The steep descent makes the fuselage and tail want to tuck nose down, with the forward moment provided by the horizontal tail. To stay level, some aft cyclic is necessary to balance the moment. This aft flapping could result in the rotor wash having a forward bias, requiring some slight forward speed to keep it reingesting.
That is pure speculation, but it has a certain ring to it, I think.
Nick