cst simulation open add space boundary condition

i am simulating a structure with cst , my boundary conditions are all open add space . when i simulate my structure in frequancy range btw 2 to 6 gh i get totally different result than when i simulate the same structure in frequancy range 2-10 gh . so , what is wrong

when u set the frequency 2-6 ghz the mesh size is set acoording to wavelength at 6 ghz, if u set the frequency 2-10 ghz the mesh size is set acoording to wavelength at 10 ghz. if u didnot simulate in adaptive mesh this differency is expected.
to not see the difference and to achieve to accurate results u should simulate in suitable mesh size