Problems in CST： Frequencies in Port1 and Port2 are not compatible

Hi Dear Members,
I have some problems in simulation of a Horn antenna in CST.
Frequency range: 27.9-28.1 GHz
R_port1= 7.15 mm
Result:
operating frequency at excited port1: 29.4 Ghz
operating frequency at output port: 28 GHz
This is my model

What happens with operating frequency of wave at excited port1 ? Why 29.4
experts plz help.

Thanks in advance

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