passive mixer-first receiver IIP3 simulation: IM3 curve not 3dB/dB

Hi,

I design a common passive mixer-first receiver(passive mixer +TIA) in TSMC65LP  CMOS process. In the test bench, I drive the receiver with a port , then a idea_bulun , then an ac coupling capacitor connect to the receiver. I use HB method to simulate the IIP3 of the receiver(Lab 10: IP3 Calculation (hb with Three Tones)  ). I have two questions: 

1. why the IM3 curve is not 3dB/dB ?

2. when I set different "number of harmonics"（from auto 3 3 to 3 2 2）, the IM3 curve change greatly, but the slope is still  non-monotonic. 

Why is the difference so significant and how can I simulate the correct curve ?

Thank you!