Why port impedance vary with the distance from discontinuity

[d04500]

:?:
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?

:arrow:
HFSS 10.0

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

Added after 6 minutes:

HFSS design file attached...

Added after 19 minutes:

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.

 

[d04500]

Anybody can help?