Depending how you drive, a possible explaination for the lower consumption at min marks goes like this: The largest part of consumption is through oil carried through the crankcase breather system into the intake (hose at top of valve cover). If you are driving and stopping in a way that gets oil on the rotating crank a larger mist of oil is carried with the crankcase vapors.

The idea behind the max level is to keep such activity to a minimum. That motor takes near 8 quarts and has an oil cooler so I can't think of any reason why it couldn't be run forever two quarts down if it helped consumption.