I cannot provide at the moment exact answer but just above the table in the FCOM states :
“The capacity of the units is such that the aircraft must descend and remain below the following profile.” Is this a certification of the O2 units based on the worst possible scenario that could exist on earth ? (Emergency descent over the Himalayas ?) just a thought .
Another thing that puzzles me is that since the reaction to produce oxygen for passengers is chemical ,you could expect that it should last a defined period of time , but apparently it doesn’t . Could it be the less ambient oxygen in the cabin the faster the reaction is , thus lasting less ? That could be the case actually , purposely designed like this ....once you descent to a lower altitude the oxygen demand for a living being is less thus the unit can sustain you longer