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[Scatty]

The computer opponents receive periodically every now and then moderate boosts of mana which they probably spend in exchange for gold and to upkeep and summon their units. If you ever saw the weakest of the opponents who's laughable at best positioning 5-6 sky drakes or 5 death knights in their capital city you know where the wind is blowing.
It is cheating indeed, cheap replacement for the lack of good AI, but that's the way the game continues to stay challenging (a bit at least) and interesting anyway. Noticed the same thing already long ago in Transport Tycoon, seems to have been a usual tactic to program the computer opponents back then when making a good AI would otherwise make a huge game.