If we don't try to solve the question "in general," specifically in relation to "Heroes": are there players who are equally good at playing any faction? And can such a versatile player lose to a player who is more skilled at playing only one or a few factions?
1) Their opponent is exceptionally good at playing their faction.
2) The master is slightly unlucky, or their opponent is lucky (the element of luck always remains, in any case).
Where do we end up when we save the world?