I was a bit confused as well, would it be that the skin is a good barrier to the radiation, so most would be deflected away, meaning you would only 'absorb' a very small amount?
But rads are a measure of absorbed radiation. However, rads don't distinguish between different types of radiation like sieverts do. So 400 rads of alpha particles (hardly penetrating skin at all) have a very different effect on the body than 400 rads of gamma radiation (messing things up in deep tissue).