Please tell me what I am doing wrong.

Pls tell me how exactly the elo System works? I have a 70% winrate for the last 20 games (56% overall) and acording to my mmr is about silver 1, I am ranked in silver 2. So far I´ve played 2 games today. My stats for the first one were 13/1/0 (well I could have had more assists, but you know I´m a Darius main so ....) and I also got a Penta that game. I got 18 lp for that. The next game I went 8/7/9, not good, but I wasn´t feeding. EVERYONE ELSE in my team had 12 deaths. 12!! Our Yi had a lot of kills, but was still useless and botlane was just inting the entire game. I didn´t play well, but I managed to survive quite well. I lost 20 lp for that game. 20! I am just so confused. I used to get 22 lp per game for like 10 games in a row, then it just stopped and I started to get 18 lp per game. That was during a winingstreak. The lp I gained changed from 22 to 18 without losing. I mean I lost later, but I started to get less lp before that. Oh and btw I am not sure if I used the right board, pls tell me which one I should have used, if this indeed is the wrong one.
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