The rotor design (in this case mostly its local pitch angle, set by twist and rigging) is a compromise. For ideal autorotation, the twist and collective rigging could be one way and for hover efficiency quite another. The bottom collective is set by the ability to successfully enter and stay in autorotation, usually at the worst case state for the approved envelope. Worst case is light weight, low density altitude, where the trimmed steady low-collective auto rpm is lowest. If the aircraft is mis-rigged and the bottom pitch is a bit too high, the auto rpm might slide below the green arc. Why not have massive collective pitch range to allow lots of bottom pitch and also lots of high collective? This is one of the most expensive and heavy items in the designer's tool bag. Collective pitch range is also main shaft (mast) length, and also servo length and collective control rigging. Masts and servos and controls are among the most expensive and heavy components, so every degree of collective range is hard-won.
Many helos have a maintenance check to confirm proper rigging, others have enough margin in the rigging procedure to assure same without actually checking.
Naval helos that have to glue themselves to a tossing deck often have a few degrees of negative collective pitch so they stick themselves down. You can tell this by the fact that as you land, you can watch the torque go back up as the collective gets to the bottom.
These naval machines have the strong propensity to overspeed in auto, of course.
What affects auto rpm? Gross weight, the higher the weight, the faster rpm builds and trims. Density altitude, the higher the altitude, the faster the rpm builds. Airspeed, the faster the speed above the Vy, the less rpm the aircraft trims at.Pitch attitude rate, a push over (bunt) causes a strong rpm decay while a pull up causes an rpm build up.