Different S11 values for the same frequency

Hi all,

I simulate a pin-fed patch antenna at 3.5 GHz. When I set Frequency to 3.5[GHz], The S11 value is -28.474 dB. But if I enter range(3.3[GHz],0.1[GHz],3.7[GHz]) phrase into the frequencies text box. I get the following results, which is -23.569 dB at 3.5 GHz.

3.3000  -3.9214
3.4000  -8.0754
3.5000  -23.569
3.6000  -10.408
3.7000  -5.3938

Why are two S11 values different at the same frequency (3.5 GHz) even I don't change anything else?

Thanks.