Normal quite simply means looking like the majority of wild Ball Pythons. Its also called wild type. The part that you are missing is that there is more than one gene that makes a BP look normal. A mutation in one gene will make an albino a mutation in another gene will make an axanthic (having two of the mutant genes in this case because these morphs are recessive). Because these mutations are for different genes (alleles) you can have a morph that has more than one mutation like an albino and axanthic makes a snow.
So to put it another way. A spider has the mutant gene that makes it a spider but it has the normal genes at other alleles so its not an albino or an axanthic.