Different Ant Impedance simulated CST & HFSS

Hi all,
I am simulating a similar design in HFSS and CST (TD) but getting different impedance. Any suggestions why ? which one would be more close to the real one?
Thanks

my suggestion is to NOT trust ANY simulation program, and get out into the lab and measure a real antenna!

I agree, but HFSS has predicted real results in the past and number of times but am not sure about CST.