It is the mass of air we breath that matters
The mass of air that is available in the cabin, as soon as the doors are closed, is mostly at the contol of the crew. (FD).
I agree 21% proportion does not change dramatically at sea level, but taking an extreme example there is no O2 in space, or even at the top of a mountain there is very little O2.
The aircraft cabin is designed to be at 8000 ft or similar, and the breathable O2 is less than at sea level.
The partial pressure is less, and thus the effort needed to actually get sufficient O2 from the air is greater. Thus slightly self defeating, less O2, and more effort, to get the same mass of air into the lungs.
Also the air in the cabin is drier, and has a slighly higher percentage of Ozone. Less mass air flow, and usually warmer that one would like.
When driving to the airport in your drop head E Type, cold wind blowing thru your hair: never felt tired. NO, never.
All these go to make the feeling of sleepyness, thats why we turn the air off at night, to save fuel, as to make fresh air the burners are on all the time, and secondly the lights are off to save electricity.
Yes the cabin pressure CAN go up after the doors close, and CAN go up as power applied, depending on aircraft type, and more importantly TO Config bleeds.
Anything to save a few cents.
glf
Tightslot: it was SOP on all return fligfhts, in my day, did not know it was the full rotation.
The French are leading the way forward in in fliight theft, and its not just the charge for the use of the head rest. I think MOL should install seat back safes.