I think betting markets are prone to wishful thinking; there's no guarantee that errors of reasoning toward one side or the other will cancel out, since it's perfectly reasonable to imagine that errors in one direction are more common.
There's no guarantee, but the bookie is watching his, uh, book and adjusting the odds to make sure the payouts come from losing bets rather than his pocket.
Not on Betfair, it's an exchange. They make money either way.