What are different "strains" of the same apple cultivar?

Many cultivars of apples have multiple strains or selections which will have different names usually in front of the cultivar i.e. Grand Gala, Gale Gala, Fulford Gala, etc.  These different strains are all of the same cultivar with some unique characteristic usually relating to color, size, or maturity.   The availability of different strains is more prevalent in some cultivars than others.