Looks to me they are just using the average win rate of the top 250 on 1v1. Which is as useful as an ashtray on a motorbike.
Having win rates over 50% for all the factions doesn't indicate balance at all. What it does indicate is that the matchmaking is whack if people in the top 250 have win rates over 50%. It means people aren't evenly matched most of the time.
