I was wondering the same. I presume the human colony rebels (whatever?) are influenced by the difficulty and they make the game easier since they only help you.
It's not as though having a higher difficulty will result in a net gain in "easiness," because the AI will gain more from the higher difficulty then any allies like the rebels or Dyson sphere would.
Also, the Dyson sphere spawns 1 Gatling ever 20 seconds regardless of what the difficulty level is, so I assume AI difficulty only effects the Gatlings personality, or has no effect at all.