@mac64
You answered your question yourself
.
If two divers enter the water as a buddy pair. One on Air (Nx21), on on Nitrox 32 (Nx32). Both dive with the computers set for air.
Dive Option 1
One diver is breathing 79% Nitrogen, one diver is breathing 68% Nitrogen.
When they leave the bottom following the air profile, the diver using Nitrox has less nitrogen in their system (tissues), than the diver on air. Hence the diver using Nitrox has a safety buffer.
Dive Option 2
One diver is breathing 79% Nitrogen, one diver is breathing 68% Nitrogen.
The diver using air dives with the computer set to air, does a square profile and follows the NDL dive time, surfacing
at the limit of the NDL.
The second diver (Nx32), does the same square profile, with his computer set to Nx32. He files the NDL dive time, surfacing at the limit of the NDL.
The computer (Nitrox table) gives the second diver a
longer dive. When he leaves the water, he has the same nitrogen saturation as the air diver had when he left the water at the end of his dive.
The two options produced to different risk factors.
Option 1 gave the Nitrox diver a 'safer dive' (with regards to DCI), than the air diver. Because although haveing the same dive as the air diver, his had less nitrogen in his system at the end of the dive.
Option 2 gave the Nitrox diver the same 'risk' of DCI as the air diver, i.e. he dived to the edge of the NDL, but he had a longer dive.
The air diver could reduce his risk of DCI, if he choose to ascend with 10minutes of NDL left on every dive.
The Nitrox diver could follow the same practice, choosing to ascend with 10 minutes of NDL left on every dive, he would have the same risk as the air diver, but longer dives.
In the above example we are assuming a perfect profile to the same depth for each diver. (i.e. how a table is calculated.)
To my view, diving to the edge of the NDL is aggressive diving.