As far as I know, X-COM does it the other way round. The harder the difficulty the more the rolls are rolling in favor of the AI, only on the lowest difficulty I think the rolls are in favor or the player.
As long there is no real artificial intelligence, the AI will always be dumber at the end, so they most needs an advantage to remain a challenge.
I'm not so sure about the enemy ai being dumber, considering that their pathfinding has no troubles at circling over elemental fields while my characters' pathfinding always dives right into them
