Calculation Gain in LNA

I want to obtain gain in LNA
1- I used S-parameter simulation and I get S21
2- I used AC-parameter and got 20*log(mag(Vout)/mag(Vin))
3- I used HB-parameter and got dbm(pout)-dBm(pin)

why are all values different?

S21 that is small signal staff and less or more precise, HARBEC? HB parameter nonlinear and probably mainly exist for dynamic P1 OIP3 an so on. And what is AC parameter? what do you mean.

In AC simulation I used this formula :Gain=20*log(mag(vout)/mag(vin))