Many thanks, S&L!
Something 737NG pilots may not be aware of, is that, with the APU running, there are two forms of high flow on the ground... The one you described in AUTO mode and the manually selected HIGH. The latter forces even more air into the ducts.
Different solenoids in the Pack Flow Valve are used to force the pack into these modes.
The normal AUTO "high" air flow (with both engine bleeds switched OFF) is 80 pounds per minute. Normal AUTO Low flow is 55 pounds per minute by the way. The manually selected HIGH (on the ground with APU running) is 100 pounds per minute. Selecting "HIGH" with only the engines providing air or with the aircraft in the air only produces the first mentioned type of high flow (80ppm).
Regards.
Q.