measured and simulated S11 of a patch antenna

dear all
in the following figure, anyone tell me why is the measured S11 larger than the simulated one in low frequency but smaller than the simulated one in the high frequency

They look comparable .

but we notice thet the measured S11 larger than the simulated one in 2.4GHz but smaller than the simulated one in 3.7GHz ?

At these S11 levels of < -20dB, small changes in antenna impedance make large difference in dB(S11). Compare in Smith chart, and you will see they are very close.