Why port impedance vary with the distance from discontinuity

It's said port characteristic impedance can be defined only on cross section parameters, isn't it?
But, my practice says no.
Any one can help me?


HFSS 10.0

microstrip: PP thickness 4.2mil, Er=4.1;
Trace width 5mil.
discontinuaty: trace with 20mil-width.
distance from discontinuity: 200mil~500mil.

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HFSS design file attached...

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I'm wondering if it is a software bug?

HFSS runs quickly on "solve ports only" mode.
My feeling is it solves 2D on port cross section area.

Anybody can help?