Why do I win more playing in higher SR games?

Im about 2300 on PS4. I usually play Pharah, tracer, mercy, or lucio. When I solo queue obviously games are around 2300 average. When I play here I seem to win sorrel above 50% of the time.

However, I duo with my IRL friend. He has less time to play than me and got placed lower so he's at about 1600. When we duo the average team SR is between 1900-2000 and it seems like we win about 33-40% of games.

At the lower SR, I'll almost always play a "carry" DPS unless we really need a mercy. My buddy plays Zarya or torb or DVa.

I'm really at a loss as to why I win more at higher SRs. I don't think my friend should be lower than what he is, in fact he probably plus closer to 1900 or so. Does anyone have any suggestions as to what might be going on or what I can do to identify why we keep losing so much?