My gut feeling is that, just as at the top end of the power curve where you can sometimes gain more efficiency by lowering Nr (beeping down) and pulling more pitch to compensate, thereby getting the blades to a more efficient angle of attack and therefore saving fuel, you can do something similar in auto-land.
Light aircraft, bottomed collective to maintain auto Nr, get a certain rate of descent.
Heavier aircraft, holding some pitch to control Nr in the green, blades are now at a more efficient angle and therefore ROD is lower.
No maths there, but it sounds OK to me - critiques welcome.