ADS simulation doubt ampr

in ads schematic I placed an amplifier and imported S parameter(from an .s2p file) to the amplifier and did S-parameter simulation after doing 50 ohm terminations.I got S21 as in data sheet.Then I placed an amplifier in schematic and imported s parameter(from an .s2p file) and placed a 50 ohm as load and and connected P_1 tone source and did ac simulation in same freq range and plotted 20*log(mag(vout/vin)).But it is different from S21 plot in data sheet.why is it so?How can measure gain in such a set up?

vout:o/p voltage of ampr
vin:i/p voltage of ampr

thanks in advance

Have a look!
https://www.edaboard.com/thread111652.html