About ADS model

I have designed a LNA using ATF54143, and simulate it using ADS model(download from www.agilent.com ). It works well now. But I found that the simulation results based on the ADS model(ATF54143) is not very close to the fact value,as though their variety direction is same,and the simulation is valuable. So I don't know how the ADS model work, and what problem result in this.