Page 28 explains that it's based on which animals appear to suffer:
Hypoxia produced by N2 and Ar appears to reduce, but not eliminate, aversive responses in pigs. [...]
Advantages—(1) Nitrogen and Ar do not appear
to be directly aversive to chickens or turkeys, and the
resulting hypoxia appears to be nonaversive or only
mildly aversive to these species. Similarly, N2 and Ar
gas mixtures do not appear to be directly aversive
to pigs and appear to reduce, but not eliminate, the
behavioral responses to hypoxia. [...]
General recommendations—Hypoxia resulting
from exposure to Ar or N2 gas mixtures is acceptable
with conditions for euthanasia of chickens and turkeys. Likewise, hypoxia resulting from Ar or N2-CO2
gas mixtures is acceptable with conditions for euthanasia of pigs, provided animals can be directly placed
into a < 2% O2 atmosphere and exposure times > 7
minutes are used. Use of Ar or N2 is unacceptable for
other mammals. [...]
As an aside, I wonder if a PCB (Penenetrative Captive Device) has ever been designed for humans, considering it is recommended over firearms for animals.