Well Sylvanas and Nathanos have a thing going on...
Generation are a thing. Yes, they are not called generations in lore, they are called Necrolyte Knights (gen 1), Fallen Knights or Fallen Paladins (gen 2), Death Knights of Acherus (gen 3). As I said every gen is different with different strengths and characteristics, also the process of creation of different generations are different (f.e. gen 1 have free will, gen 2 are not actually undead,...)