s parameter problem on cst 2010

everytime i changes frequency range, s1,1 curve also change......
why this is so? tjis create problem in getting the exact result....

What is your center frequency?
How about fixing center frequency in your setup? I think that CST will use the center frequency for meshing. Therefore, if you change it, the result will change a little bit.
For example, your frequency range is 100 - 300 MHz. The center frequency is 200MHz.
So, just change the frequency range like 50 - 350 MHz, and so on.

That error may be due to the step you choose between each run. Example
First run: 1GHz - 3GHz with step 1000 but after that it is rechanged step to 500.

Changing the frequency of simulation leads to new mesh-cells for your structure, so your results can be changed!