Not variation, just different generation of bolt carrier. My Bushmaster from '01 had the notched out firing pin area, and the '05 CMT bolt had it filled in. I've had no trouble with either and don't think it really matters. I personally like the notched carriers as they're easier to clean :) I suspect the newer design might be a fraction stronger since there is more material, but I highly doubt that's why they made the change...as I've never heard of bolt carriers breaking (unless they were a manf defect).
IMHO, It's probably simply just cheaper to machine the carriers without the notch. Nothing more, nothing less.