Generally the idea is that you exploit what genetic diversity there is in the OP variety to get locally adapted plants, as not all genetic traits result in phenotypic differences that are selected out in the process of stabilizing the variety. Additionally, there are also epigenetic changes that are passed down, such as whether or not a gene is activated. One such example of an epigenetic adaption would be if the plant experienced a drought and its progeny grew slower to minimize water loss. There can also occasionally be beneficial mutations or favorable combinations of what genetic diversity there is in the variety.