LongTime - I do agree that there must be some variation with relative heights between tank and the suction device at the tap. My hose length is constant, so as long as it is not crimped that is constant. Vacuuming the floor level tanks is slower than the top tier tanks - which comes as no shock to me, but I assume it averages out as the tap is just barely lower than the top of the middle tier of tanks.
I would suspect their tests were done at zero head either way, and have not gotten any response asking for more clarification on test conditions.