Need help to explain the difference between S11 measurement and simulation of a coil antenna
A few things are important:

First of, what do you mean "the \$S_{11}\$ is -2dB"? The S-parameters are frequency dependent. What frequency is this at? Usually, S-parameters look something like this (own work, from a filter my team designed):



About the actuall difference between simulation and practice: This is the life of a antenna designer/microwave engineerin. Take a look at the following filter designed and simulated in ADS/momentum, and then produced on a high-quality process on high-quality substrate:



Here too, we see a shift in center frequency. This was eventually traced back to underetching in a set of slots in the filter. This was due to inaccuracies on the order of micrometers! 

So, what could cause this in your case:

When you did your simulation, did you correctly simulate the conductivity of your conductor, the wire thickness, etc, or did you assume an ideal conductor?

Did you verify that the impedance of the source connected to the antenna during measurement is the same in simulation as it is in practice (over freuqencies of intrest)?

When you measured the practical \$ S_{11}\$ measurement, did you make sure that the antenna was not on a surface, and sufficiently far from everything that could couple and change it's performance (A antenna changes if you put it on a dielectric, or even worse, bring a conductor in its near-field). 

How sensitive is your design to manufacutring tolerances? In your simulation, your antenna is likely a perfect coil, but is this the case in practice? How much impact does this have on your resonance frequency? The antenna is a long coil - what happens if someone slightly bends this coil? Have you considdered these options?