- Global balance as a sum of local imbalance
Balancing a game require only that the states "player 1 has won" and "player 2 has won" have the same probability to be reached. Everything from the start to the next-to-last states could be utterly imbalanced. For instance, Blizzard has found during the Starcraft 2 beta that in Terran versus Protoss (a subgame of starcraft), protoss tend to win 70% of the time when they got a (pack of) powerfull units called carrier. For instance, if the state "Protoss made carrier" occur 10% of the time (my hypothesis), it means protoss achieve victory 7% of the time because of carrier, and Terran only 3%, which is +2% in favor of protoss. If in the remaining 90% of the game, protoss win 43% and terran 47%, the game would be balanced, according to blizzard's statistics (and my hypothesis).
This graph illustrate the idea of local imbalance : protoss has 70% chance of victory from the carrier node, still only 50% from the start. Probability (in percent) used here mean some degree of uncertainty more than randomness.