I think I understand.
Nitrogen and Oxygen act differently.
You have a hard limit of 1.4 (or whatever you want) because O2 goes toxic at some point. One starts to see some deaths, and an increasing number with PO2s over 1.6 and a few at 1.6 with activity.
With Nitrogen there is not a hard limit other than the higher the PO2 the quicker you will have to do deco on the way up. More nitrogren may narc you more but it does not put you into convulsions which result in drowning.
In addition, we actually DO set a limit on PPN2, it is about 3-4ATA. This is the limit many people use to avoid narcosis (for air that is about 100 or 130 ft). The analogy to O2 toxicity limits of PPO2=1.4 is pretty good; in both cases it is the partial pressure causing the problem, not the depth or the time.