Question about Ranked

I had been promoted to S2, lost 5 games at 0 lp and got demoted to S3. I go through 3 different promos to get into S2 again, only to get back down to 0 lp. Keep in mind, I was winning 18-21 lp per win. 1 loss at 0 lp demoted me down to S3. I don't understand how 1 loss compared to 5 losses last time demotes me when I was 0 lp for 1 game. My MMR was bad before where I was only winning 15 lp per game, but playing more I got it up to almost 20 if not more per win. How does this work?
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