Why does the imaginary part of Z11 change with the impedance of the input port

[xiaowenrun]

Hello everyone
I am doing a simulation using Cadence Spectre. And I found that the imaginary part of Z11 changes with the impedance of the input port. Does anybody have any idea about it? Thanks in advance.

 

[BigBoss]

Why not ??? Z11 is essentially Input Impedance while the Output is Open Circuited.