Logically what you are saying makes sense, when you calibrate using test tones the receivers master volume should be turned way down, then each speaker would be moved up in db to reach the desired level.
If the master volume is turned way up during the tones each speakers volume level would have to be reduced to hit the desired db. Thus leaving you less headroom to raise the volume. That is at least my take on it.

A computer once beat me at chess, but it was no match for me at kick boxing.