Sparameter V.S. Output spectrum

I am simulating the conversion gain of a 2.45GHz LNA by ADS.

1.Find out output power(dBm)@2.45GHz from output spectrum
then substract specified input port power(dBm)

2.S21 by S-parameter analysis

3.Measure Vout and Iout @2.45GHz then calculate
PowerOut(W)=0.5*real(Vout*conj(Iout))
PowerOut(dBm)=10*log(Power(W)) + 30
Then substract specified input port power(dBm)

Should they be the same?

Typically I use 1 and 2, but an ADS Design Guild example use 3
I got three different answers.
Did I do anything wrong? Shouldn't they be the same?

Does anyone know what the difference is?
What's the typical way people used to simulate power conversion gain?

Thanks!

If the LNA is not in compression when doing the Power Gain (Pout ? Pinp), this should be equal with low signal gain (S21)
I don?t really understand what you are doing at measurement nr.3