Tell me now that Riot doesn't predefine Win/Lose in ranked games

So, can anyone explain to me, how a Silver 1 with less than 50% winrate (so definitely not high MMR) player gets into a game with avrage rank Gold2 and yet somehow enemies have a Plat player with 50% WR. How is that a fair matchmaking. That doesnt make any sense at all. Neither Seju nor Jax were premades. Jax has Plat 4 in flex, but he is getting carried every game based of his KDA performance. Not to mention that he got fcking destroyed on lane, unable to make decisions on similar lvl as rest of the players.
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