I think you hit the nail on the head with this one. I think we should get a hidden "major faction bonus" based on difficulty level for all playable factions except for the one chosen by the human

Food Bonus:

+5/turn on Normal
+10/turn on Hard
+ 2 per settlement on Very Hard
+ 5 per settlement on Legendary

Income Bonus:
+500 on Normal
+1000 on Hard
+ 1000 flat + 150 per settlement on Very Hard
+ 1000 flat + 350 per settlement on Legendary