Luck in matchmaking
Does NOT play the same role for everyone. I get afks/connecting issues with allys, junglers who don't take smite, supports who don't build sightstone, lanes that feed 10 deaths in 10 minutes, I get matched against diamond smurfs (who add me after games to prove it) and players who lack very very basic fundamentals that make their every move totally unpredictable, one game after the other, NO dispersivity what-so-ever. Just a luck based losing streak over a small sample size that completely fucks up my MMR and winrate.
Ya know, I thought the longer I played this game and the more I improved would actually reflect the types of teammates I get on my team, players who actually go into a game with an idea of how any given matchups works for/against them, players who spend hours honing basic skills like wave control, trading and split-second decision making, and I thought I did until I got over a 65% winrate and realized I was getting matched with non-competitive, low calibur players who have no idea what teamwork means so riot could "fix" my winrate.
This is the reality of league people. Riot has fixed this system so you NEED to play a massive amount of games to even climb, because when you're good enough to climb fast, the system rigs itself to knock you down preemptively because you haven't "played enough games".