CST - e-field not symmetric of the coplanar waveguide

Hi, when I use the frequency domain solver with tetrahedral mesh to simulate a coplanar waveguide transmission line, the e-field is not fully symmetric (the setting of waveguide port is symmetric). Why is this? Is it because the tetrahedral mesh is not symmetric or not dense enough?

                                                                 e-field of port 1 along u-axis


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