The rating is a representation of your skill level in Battlegrounds which aids the developers match you to like-skilled players to ensure fairness,
“In the way we designed our rating system, on the second day of Open Beta, if your rating was 4200, you were roughly more skilled than 77% of the players playing Battlegrounds; if your rating was 5000, this would be better than 99% of the players.”
Additionally, the rating updates after each match, which introduces variance. Variance is a metric which is associated with your rating, but isn’t presented to you the player. The variance represents Blizzard’s confidence in your rating level. For example, the more matches you play, the higher Blizzard’s confidence in your rating, and the lower your variance.
Your rating is affected at the end of each match by a combo of your rating, your opponents’ rating, your finished place at the end of a match, your predicted win chance, variance, and “a few other factors.”
Additionally, some recent changes were made regarding how your rating is calculated,
“We want to ensure the ratings don’t swing too wildly and end up putting people in the wrong rating groupings, and we also want to ensure that you are getting closer to your correct MMR each time you play a match. We increased your rating gains and losses once your rating has been established when you’ve played enough games so you will see more movement in your rating.”
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