Chris, I'll add that the built-in test tones on receivers(and some separate discs)are 30dB rather than 20dB down from the full-scale 105dB level, so the calibration for reference in that case is done at 75dB rather than 85dB.
Then, as Chris Neon points out, the "0" level on the main volume control doesn't mean the same voltage output is occurring in all cases. The automatic or manual calibration adjusts the voltage output at 0 to take into account all the factors you mention so that the desired 75dB or 85dB is achieved at the listening position.
