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Actually it was 1771 to 1788. Doesn't make any sense that they would win by that much taking more battles to clear my teams.

im not sure if it would make up for the 2 extra battles but im pretty sure you get extra banners from using undersized squads,  do you think it might be possible that your opponent used multiple undersized squads to get the extra banners?

Did a forum or google search not give you the answer?  This question comes up quite a lot, including in the last few days.

So 1771 vs 1788 is in Kyber.  1771 is not a great score for a full clear; I'd expect to be beaten with a score like that.

What you haven't written is how many failed attempts there were on both sides and how many repeat fails on certain battles.

As you've both clearly failed quite a few times (I hope you've failed a few at least with that score) it could have gone either way, depends who was more efficient; in this case it was your opponent.

*After the second failed attempt vs a team, you can't drop any more points.  So, a player could drop 7 battles vs 1 team and lose far less points than if you dropped 1 battle vs 2 different teams.

*You get a set amount of points for a win.  You drop points for a first lost, drop half that amount for a second loss.  You also gain an extra point for; empty slot, full protection, full health.  You lose a point for each death on your side.  For character battles; you can score as little as 16 points for a third attempt, where you lost 4 characters and the only surviving member doesn't have full health or protection.  You can score as much as 69 points for taking in 1 character and winning a first attempt with full health and protection.  As you can see, that's a big difference of points for a win.

In this case, especially if they understand it had come down to efficiency, it would be quite manageable to make up that points difference.  The highest score you can obtain in Kyber is 1905, OK you aren't going to see that, but scores over 1850 for a clean run is something you're likely to see.

In short, almost certainly not a bug.  However, wait until tomorrow or the next day and review it on swgoh.gg