ppf calculation

there is difference between circuit simulation SNR and calcultion SNR at the output of receiver's poly-phase filter(PPF) output. The simulation SNR is higher about 3dB. I can't understand why it behaves like this. the SNR is the same whether you use RFDE simulator or spectreRF simulator. Could anyone help me ?

I think there is an option to define either DSB (double sideband) or SSB (single sideband). That give typical about 3dB difference, not exact.

Then How to understand it?
Would you please give some detail information?

I think image noise will add to the desired signal if Polyphase filter didn't work well.So the SNR will degrades 3dB from Polyphase filter.