It looks like you have included a lot of attenuation in your measurement, as well as the S11 of the antenna, so it looks like the S11 of the antenna is better than it really is. If you have 10 dB attenuation each way, for instance, such as in the cable to the antenna, then S11 will look 20 dB too low. If you have attenuation like this, you need to measure it so you remove its effect.