They must have been playing some other game that ended while you were playing. OK that's just a guess, but it's probably what happened.
Ratings changing mid game?
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It does seem a quick rise though? I try and choose a game where I have a similar or slightly higher rated opponent so it's quite frustrating to find I'm playing a genius after 2/3 moves!
Remember the default rating is 1200. So once a new "unrated" player finishes a game their rating goes up/down from 1200.
How does it happen that I start to play someone from an open seek who is 'unrated' and then after a few moves the rating has suddenly gone up into the 1000s?