CST results question
As far as I know, conical horn antenna produces linear polarized wave if excited by TE11 with certain polarization. If you want to generate circular polarization, then you need to excite it with TE11 of two perpendicular polarization with phase shift Pi/2.
I think in your case the antenna is producing linear polarization, which may be decomposed into two equal magnitude circularly polarized waves one RHCP and the other is LHCP. I think CST does that and that is why you get the same pattern
Best Regards,
Adel_48