How did he win?

[Incredipede]

The winner definitely started with the illegal formation and changed it during the game. It is really the only explanation. That and he got lucky as 3-5-2 is a solid formation. Without being there to watch the game, you really cannot ever know, unless you look at the actual players and their positions, which we cannot do with screenshots.

The fact that you really outweighed him quality-wise, just means that its a normal unlucky NGE outcome...although I usually see this happen more often for visiting teams than home teams...

If I start a match with 3-2-2-2-1 Butterfly and change to 4-5-1V while the match is running, the statistics after the match say that I've used 4-5-1V, not 3-2-2-2-1 Butterfly. It looks like the statistics show the last used formation in the match.

This means that this guy could have changed to an illegal formation in the last minutes of the game which would make sense. It could be that he had 47% possession before and because of the illegal formation in the last minutes of the game, the possession dropped to 43%.

That's what I experienced, but I could be wrong. I'm not exactly sure that it shows the last used formation but my brain says so. However I got proof on a game, I started with 4-1-3-2 and then changed to 4-1-2-1-2 Narrow Diamond, it shows 4-1-2-1-2 as used formation.