that's because some leopard gecko traits are polygenic, and can be replicated as well as improved further by linebreeding. in balls, there's no guarantee that certain traits (usually pattern or color inensity) you like will trickle down to the offspring. a clean and reduced pattern pattern can produce "dirty" offspring and vice versa. so my rule of thumb with ball pythons is to buy what i like (not what others like), this way i get to enjoy the snakes while waiting for them to grow big/old enough.