The map says "highly developed" and that together with Amendola's image of Vathlo always made me think they were highly advanced people. I don't know if they were more advanced than the people in Kandor or Kryptonopolis. I just like to imagine they were.
You got to recognize that superficial differences emerge when groups of humans are separated from each other for extensive periods of time. It takes thousands of years for these distinctions to appear. So first you have separate societies that have almost no contact with each other.
If Kryptonians were always cosmopolitan then there wouldn't be any distinct difference between them because they would all live in the same melting pot where any great variations would never develop.
Obviously, if Krypton is a giant planet, then it becomes more difficult for people to constantly be mixing with each other, becasue of the distance they have to travel. One can imagine different ethnic societies emerging during long epochs of separation.