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.
This is also very true.
It's hard to balance out roll bias, especially how that bias is effected by difficulty.
I'm unsure where I stand on that issue completely, honestly.
EDIT:
In the end, a slight bias in favor of player in classic but changes to slightly in favor of AI in tactician? Mmm, we want to keep AI challenging by giving them as many advantages as possible without destroying the illusion of fairness
