Comsol multiphysics (frequency range)

I am working as a beginner with both CST and Comsol multiphysics.
my model is a coaxial pipe with 4 small coaxial pipes around it .
some thing strange I got with Comsol.
in CST the frequency range doesn't affect on the S-parameters (S21 :transmission trough one of the small coaxial cables)
while in Comsol by changing the frequency range we get different S21 . which dosent coincide with Theory .
as I know the frequency shouldn't change the transmission much.
is there any idea about it?
thanks