http://i81.servimg.com/u/f81/11/20/39/11/image10.jpg
I tried to figured it out, but I have no clue about the matter.
FYI, I am level 2 at the moment. I also have a same aged DC, and he gone above 35 in defensive skills. Is it maybe because you can't have 2 different skills separated by more than 20 levels (as here, 15 DEF < 35 OFF) ?