Rakan Expectations

Using the Imgur post above you can see a 20 minute game in which I went 1/2/29 as Rakan, I was expecting an S/S+ but I got an A. The average KDA of a Rakan is 2.3 / 5.4 / 13.2. I would like someone to explain why I got an A going 1/2/29 as Rakan.
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