I have used the symmetric boundary conditions to solve only portion (upper-half) of the waveguide, please see the below figures

I have got some reasonable results but the dimension of the designed wavegyuide are different after FDE solver as shown below. Why is it so?

Do I need to select the FDTD whole region of the waveguide (not only the upper-part)? Please kindly clarify. I am trying for the similar modes of more higher order.