Why is the difference between the isentropic expansion exponents calculated by the two methods is so large？

[FengxiangMao]

from CoolProp.CoolProp import PropsSI
import json, CoolProp.CoolProp as CP
p1 = 100
t1 = 273.15 + 20
cp = PropsSI('CPMASS', 'T', t1, 'P', p1*1e6, "methane[0.75]&CO2[0.25]")
cv = PropsSI('CVMASS', 'T', t1, 'P', p1*1e6, "methane[0.75]&CO2[0.25]")
k1 = cp/cv
k2 = PropsSI('ISENTROPIC_EXPANSION_COEFFICIENT', 'T', t1, 'P', p1*1e6, "methane[0.75]&CO2[0.25]")
print(k1)
print(k2)

yielding

1.5452628745485766
6.438960963376358

[ibell]

Because they are not the same thing. See the definition here: https://srd.nist.gov/jpcrdreprint/1.1349047.pdf