Yer talkin rocks here.
Even though it's slate and fits the definition the degree of metamorphism can vary from sample to sample as will the minor minerals so these differences and differences in particle size is what makes differences in slate.
A good example is coal. Whether it's Peat, Lignite, Bituminous,or Anthracite it's pretty much all the same stuff. Its the way Mother Nature handles it that makes the difference. And to be accurate coal isn't a mineral or a rock cause it's organic based.