/3/ - 3DCG

i have a noob question.

I'm modeling in c4d, and since i have a symmetrical object, i decided to only model half of it, and put it in a symmetry object. i also want it to be a subdivision surface model. so i have my cube inside my symmetry, and the symmetry inside the subdiv surface. why does the subdivision surfice seem to be affecting the cube, not the symmetry object?
as you can see, it is causing problems in my model, the top is not smooth, and the rounded corner in the center shows at the back part.