I know this doesn't sound as smart as it could- but from what I remember, everyone clogs up their body at different times. And I suppose genetics has something to do with it here. Some people's livers are very strong, fast, and efficient. Some are slow, very clogged, and sensitive. This probably has something to do with your metabolism, age, genes, amount of unhealthiness, etc.
It's like asking the question how do some people get fat easily and some eat whatever and don't gain anything. I think that the thin people just haven't worn out their "natural youth" yet. Oh course there's more to it. I'm 18 now, and I seem to be getting thinner over the years (lol)- I was such a chubby kid.