According to SNAP’s help PVI has PVI = sin(a) * IR_factor * near_IR - cos(a) * red_factor * red formula.
According to https://www.asprs.org/wp-content/uploads/pers/1977journal/dec/1977_dec_1541-1552.pdf (the original article where this index was proposed) it has PVI = sqrt((Rgg5-Rp5)2 + (Rgg7-Rp7)^2), formula,
or according to http://www.mdpi.com/2072-4292/4/12/3892/htm#b31-remotesensing-04-03892
it has (pNIR –α pred – b)/(1+α2)
pNIR,soil = α pred,soil+b formula.
There is no sin or cos in initial formula? Why does SNAP use its own formula?