Problem with SR

I was playing some competitive matches with a friend, me playing tank and him playing damage. As we kept playing, my friend started receiving rewards that were vastly different from mine. I don’t want to sound arrogant, but I finished almosta all the matches as top damage, with the highest number of eliminations and the lowest number of deaths. After each match, my freind and I would laugh when we found out that his hero SR (Venture) had increased by more than double mine, even thought I played about 90% of the matches as Junker Queen and was statistically the best player in the match.

The difference is so large that he has an SR higher than diamond, while I am still in silver. You can check my match history to verify this. I note that me and him have started playing at the same time and should have similar/same games on the heros.

I hope you can clarify why this happens or fix it so it doesn’t happen again. Thank you.