Difference between simulation and measured results in ADS

What are the chances that ADS simmulations does not match with the actual measured results?
Got this problem when i designed a meander antenna in layout and simulated using momentum. A very prominent resonance appears at 200 MHz, which is completely missing in the measured results...
The antenna operating frequency is 200-800 MHz, designed to resonate around 300 MHz

Hai,

The substrate dielectric constant may vary depends on manufacturer for a example if u take FR-4 substrate its dielectric constant may vary from 4.2 to 4.6 so the resonant frequency shifted to some other band.

The connector should be properly soldered.....

Some - for a 2% microstrip filter I made, I had to fabricate several copies of the design at slightly different frequencies. Of course, I was also seeing an effect from lid height, too. It does seem odd for there to be a strong resonance, though - I agree with Jamesanton that improperly attached connectors can cause some weird problems.