Chemical potentials for binary alloy

[darjaved]

I have a prim with two sites, and I am getting below for the chemical potentials:

`Possible composition axes:

   KEY     ORIGIN          a     GENERAL FORMULA
   ---        ---        ---     ---
     0      Sn(2)      Na(2)     Na(2a)Sn(2-2a)
     1      Na(2)      Sn(2)     Na(2-2a)Sn(2a)

Currently selected composition axes: 0

Parametric composition:
comp(a) = 0.25comp_n(Na) - 0.25(comp_n(Sn) - 2)

Composition:
comp_n(Na) = 2comp(a)
comp_n(Sn) = 2 - 2comp(a)

Parametric chemical potentials:
param_chem_pot(a) = 2chem_pot(Na) -2chem_pot(Sn)`

i wonder it should be like param_chem_pot(a) = chem_pot(Na) -chem_pot(Sn), why there is a factor of 2?

Is there anything wrong?