I believe it is because the 17'' MBP is wider, and the track pad does not need to sit on top of it in the 17'' model where is does on the 15'' model. I am not 100% sure this is the reason, but I know it is the same way on the 15'' and 17'' CD MPBs. (Yonah) It was discussed in one of the threads when the 17'' first came out.