hfss driven mode

HI, folks:

I've start running HFSS on diff trace.

There is only two metal traces in the model. I used driven modal analysis.
Two modes are calculated by solver.

Mode1 is odd-mode and mode2 is even-mode.
I am confused about that there could be even-mode exists. How the current return since there is nothing else exists?

pic

One thing fun is that the common mode impedance looks as if the port edge is the return path??