Let’s take a step back for a moment. The purpose of a matchmaking system isn’t to keep players at a 50% win rate, it’s to match players of roughly equal skill with each other in order to make for engaging games. The primary goal of matchmaking is to reduce the number of curb stomp games where the winners are bored and the losers are salty, because neither side really enjoys the experience all that much. This usually ends up stratifying players into skill bands, where the players within that band are generally able to give each other a decent challenge and minimize the number of lopsided games. Having a win rate close to 50% is a result of such a system but is not necessarily a goal.
Matchmaking systems can’t ensure every player has a 50% win rate because it breaks down when there are very small numbers of players within a skill band. Let’s examine an extreme example of this at work. Imagine the best player in the world, Neelo. The only player on Earth she cannot handily defeat is her rival Noriko. Between the two of them, Neelo has a slight edge and wins 55/45. If Noriko is online, Noriko can only ever be matched against Neelo and Neelo will win 55% of the time. If Noriko is not online, then no one can beat Neelo and she wins 100% of the time. There is no way a matchmaking system can ensure a 50% win rate for Neelo in such a situation.
This isn’t an issue for most skill bands because there’s often hundreds or even thousands of players playing in those bands at any given time for a sufficiently popular game. However, the very top echelons of player skill often run into issues with matchmaking because of the availability of qualified opponents. Professional competitive players playing normally online will tend to win more than they lose because there just aren’t enough players of their skill level online at any given time to match them with. Since there aren’t enough players that skilled to make matches, the matchmaking algorithms will necessarily widen the search as time passes and often match them with less skilled players often with fairly predictable results.
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