Arena MMR / CSR based on individual performance?

A friend of mine and I played the Competitive Arena playlist for both of our first times ever in Halo 5. We played all 10 placement matches in SWAT and he got ranked Platinum 1 while I got ranked Onyx 1515. I did infact place at the top of our team in every single game and always had a positive K/D, while he typically placed 3rd/4th on our team. This highly suggests that CSR is NOT only gained for winning, contrary to what the internet says. Does anyone know of an explanation or some insight?

Bonus question: If CSR is in fact based on individual performance like it seems to be, then in our 11th game, why did i drop from Onyx 1515 to Onyx 1500 when we lost even though I was at the top of our team with 21 kills and 11 deaths?

Here is a link to my data:

Scrolling down to “Recent Games” on the right side will show all the placement games

Extraneous data: we won 7 out of our 10 placement matches and fellow players were always ranked about high-platinum

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