Question about impedance measurement and S11

Hello,

Thank you in advance for helping me.

I was trying to make an LNA, and I measured input impedance and S11, to make a matching circuits. (Input impedance was measured by sp sim -> ZP -> Z11 plot)

With the obtained value, I simply calculated S11dB20 and seems like the calculate result and simulation results were different (The two notches are coming from output loads)

calculation : 20log |8.37+ j1.87|/|108.37+j1.87| = -22.xx so it is quite different from -13.64

Am I missing any concepts? Or in this case what is more reliable?

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