Asymmetry in Phase for x-component of E-field at the Exit Aperture of Lens[CST]

[MFDGEB15]

I have designed a conventional hyperbolic lens made up of a dielectric material. After simulation when I measure the phase at the exit aperture of the hyperbolic lens. I can notice asymmetry in phase along X and Y. The lens is circularly symmetric so the phase should have been symmetric along X and Y at the exit of the lens. I am unable to understand why there is a asymmetry in phase arising.

CST is used for simulating this and the incidence is plane wave.

I am attaching an image of the phase plot.