Gaining/losing more Sr then people in my group

So at the begining of this season I got 2961 sr my friend got 2920 and now he is at 2994 and im at 2945. I play heals/tanks and he plays tanks/dps. We only played together so I want to ask

How did this happen?

More people play healer/tanks than dps/tank thus you’ll have to perform better on average.

As you can see out of the top 10 most played heroes only two of them are dps.

Once you’re out of plat this won’t matter anymore as individual performance isn’t used anymore in diamond and above.