S ranks

i have been playing zed for some time now and got him lvl 6. Now im trying to get him lvl 7 ofc. i only get s- on him and dont know how? can someone tell me why? all my games are good. i score poss. and decent farm maybe 8 cs / min{{champion:238}} last game i went 12/3/8 or something with 255 famr in 40 min i know this is bad but i got s- so im wondering.
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