I am guessing, and someone please correct me if I am wrong... but I think it has to do with the morphs acting like supers. And or they are allele. Kind of like the Vanilla x Fire which makes the allele Vanilla cream. Or a het red x cinnamon makes the allele gargoyle.

If I was to breed a vanilla cream to a fire I would get NO normals which is how a super would work. If I was to breed A vanilla cream to a mojave, I would get NO normals, NO mojaves and NO vanilla creams but I can get everything in between.

The special x mystic could be allele (although I have no experience with them) which means if you are to breed a mojave to a mystic crystal you would get no normals, no mojaves and no mystic crystals.