The counterweights are the clue here. This is a single acting propeller, therefore if the tank is full the spring must be fully extended to counteract an overspeed condition.

thanx dimky.will read further

basically theory says that if the oil is maxed oout the spring with its weights is not pushing much so the blades will rorate slower this is why it is underspeed.I am sorry if i didnt make you understand it well but i saw this in an image in my notes.Oil goes against the movement of the feathering spring.The spring is called feathering.If it was overspeed we would've the opposite.the spring with it's weights whould've pushed oil back (with the piston of course) and we wouldve much quicker rotation.