I suspect you have used a port at the input, and the input is completely unmatched? Perhaps you have a high input impedance with your mixer, and so pxf is giving you the gain from the internal source. Since there is an implicit gain of 2 from the voltage source, which would then be divided by 2 if the gain matched - but if driving a high impedance you'd get an extra gain factor of 2 with pxf. With pss you're measuring the voltage gain from net to net, so the mismatch doesn't matter any more.

If your mixer does in fact have high impedance input, use a voltage source as the input instead of a port.

Note, the above is a bit of a guess - without more info on the circuit, it's hard to be sure.

Regards,

Andrew.