As you get to the higher difficulties, the A.I. uses more and more advanced Algorithms to determine their moves. The most advanced ones are used when the A.I.'s intelligence is at Intelligent and above, at least according to the manual. Since the highest 3 difficulty levels each use "Incredible" as their intelligence, I imagine other bonuses are applied to actually make a difference.
Also, the A.I. does get progressively larger and larger economic, production, and research bonuses, or at least larger production and research bonuses. In the manual, it says that the A.I.'s economic/production/research levels are these:
Fool: 10% of normal
Beginner: 25% of normal
Sub-normal: 50% of normal
Normal: 75% of normal
Bright: 100% of normal
Intelligent: 100% of normal
Genius: 125% of normal
Incredible: 200% of normal
However, these bonuses are applied for levels of intelligence, not levels of difficulty... So what other advantages does the A.I. get for the last 3 levels where the A.I.'s intelligence is the same, but the difficulty is different?
Here is the answer...
I have no idea, but would love to know! Please release an updated electronic manual!!!