Well there's a couple of reasons. For starters, meth is bad for your teeth, so the people who do it a lot more have worse teeth than the ones who don't do as much. Furthermore, some meth addicts don't bother brushing their teeth, or can't afford things like toothpaste because they spend all their money on meth, where as others are able to have money to spend on things in addition to fueling their addiction. Also, some people just naturally have bad teeth.