CST-PROBLEM- Frequency domain solver-FAST S Parameters

Hi,
I am using Frequency domain solver-FAST S Parameters and S Parameters-with Fields and my S parameters change a lot when I use these.
Shouldnt the results remain the same and just that with the FIELDS; I should be abale to see the fields but the S21 should remain the same, RIGHT ?

please be more specific? as in what parameters you change, and keep constant every time you start simulation?