Yet another idea. The situation is similar to a helicopter operating in ground effect. The reduced induced flow means that for a given angle of attack, more lift is produced. You don't need more lift if the objective is to maintain height, so pitch is reduced. Thus the same lift is produced at a reduced angle of attack and the resultant of the lift and drag vectors is angled further forward, ie less drag and less power demand. In a ground effect vehicle, the whole wing operates at more efficient lift/drag ratio. I'm not sure whether the parallel of the bad news with helicopters where operating in ground effect enhances the tip vortices and tends to reduce some of the benefits. Perhaps the tip vortices in ground effect vehicles also have enhanced tip vortices.
Incidentally there does appear to be a small increase in pressure under a rotor disc as you come into ground effect (in the absence of wind) and is observable by a reduction of indicated height on the altimeter.