I'm pretty sure the enemy AIs also get bonuses to stats, XP, and/or gold income too. I just got crushed in an hour-long painfest by a team of AIs who ... well .. let's just say that with 7 kills, Oak was attacking twice a second, taking 150 damage out of the 500-600 that my character sheet said I was dishing out per hit, and running as fast as my Regulus (who had +25% movement speed). I don't know anything you can buy for the gold you get for seven kills that will do that for you, even if you also have a few thousand saved up from passive gold income, so I believe pretty strongly that the AI is getting some extra gold from somewhere.
And I haven't even mentioned the Unclean Beast, who could literally stand there and let me shoot him forever, and it just about cancelled out his passive regeneration. (did I mention I was wielding Ashkandor, Mageslayer, and Deathbringer(*)? And wearing the Bulwark of the Ages and Godplate?) I had to continually toss mines under him to even begin to damage him faster than he healed, and I could only do this standing at a health crystal, since the Beast was attacking me about four times a second (that's what it looked like, anyway) for several hundred damage each time. And that's how he was with only around 12-15 kills -- he had 44 by the end of the game!
The only reason they didn't finish us off sooner was that (a) the AI is too stupid to just walk away and grab the flag for the capturable health crystal, and (
the AI is too stupid to realize it can just walk up to my citadel and take it down singlehandedly.
Unfortunately, replays aren't working yet, so I can't find out what they bought to become so ueber-powerful or confirm my guess about gold income.
I did pull out one of the earlier rounds when it started to go this way. It was Fortress on the Brothers map, and the AI was just crushing us, killing my allies over and over, had siezed control of the entire portal side of our base, was becoming totally unkillable, etc. So I grabbed a couple artifacts from the kills I'd managed to score and took down each of the AI's fortresses in turn, plus the static defenses around them. Took just a couple minutes to work through their base, and the AI didn't bother me the whole time I was doing it. (which was odd: usually it teleports in at the first sign you're attacking a fortress)
(please note: this is not a whine, just a report on how things are. I've won every Hard tournament I played, but there are always a few rounds where I might as well have given up immediately)
(*) Deathbringer is the one mistake I know I made -- in retrospect, the Bracers of Rage would have been a lot better in this situation.