My guess is that they made the heroic difficulties not as difficult so that beginner RTS players from the Halo series will not be dissuaded from trying to get all those achievements.

So either pump up the AI difficulty and discourage achievements for beginners, or lower it, up the amount of people getting 1000 gamerscore, and ultimately increasing the lifespan of Halowars.

But just a guess.

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Oh, into the City of Woe...