Autorotation ? R22 for the Experts

Here is a good question for the more experienced helicopter pilot or
mechanic. A friend of mine that flies a late model Robison R22 Beta
II asked this question of a number of individuals and no one seem to
have an explanation. This included people at the manufacturer. I
believe I know the answer, however, I would like to put the question
out and see what others think before tainting their thoughts with my
ideas.

So here is the question: With one person flying the R22 (170lb),
simulating an autorotation by rolling off throttle, the collective
goes full down and pretty-much stays there to maintain proper rotor
rpm. With two people, same maneuver, some collective has to be pulled
to keep the main rotor from over-speeding. The amount of collective
will obviously vary dependent on a number of factors, but for this
example let's say about 1 inch off the stop. One more time with the
same maneuver, with two on board, except let the governor control
engine speed, and not rolling off throttle. In order to maintain
rotor speed the collective has to be on the stop. The question is
why? Please post your thoughts.

Thanks,

Eric D