Could both explantions be correct? i.e. a reduced power requirement occurs due to the angle of attack increase caused by the presence of the ceiling (Shy Torque) and also, for related reasons, it is difficult to accelerate the helicopter vertically because the proximity of the ceiling makes it difficult to increase the induced flow (Platinum; R1 Tamer).
You have the same problem trying to accelerate the aircraft from the hover to the ground unless a faily large amount of lever is lowered. But that isn't recommended practice
TT