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.
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.