S grade system doesn't care about assists?

So i was in a match as bard support and I went 1/4/16 and our jungler went 16/4/4 so there kill participation right there is the same, I got a B+ and he got an S- what's up with this? [](https://scontent-lhr3-1.xx.fbcdn.net/v/t35.0-12/13170551_1006332849404025_1559493274_o.png? oh=d29e334b5306137491b55a025a883db6&oe=57328809) link here: https://scontent-lhr3-1.xx.fbcdn.net/v/t35.0-12/13170551_1006332849404025_1559493274_o.png?oh=d29e334b5306137491b55a025a883db6&oe=57328809
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