I need some help with understanding how MMR works.

I get that if you win you get higher MMR, and lower if you lose. But i have played 23 ranked games so far. After placements i got B3 got to B1 with 3-4 games and the to S4 after 4 games with no promos. I got stuck there for a while i lost for games because of unlucky teammates(afk, trol etc) and i managed to skip S3 after promos. All this time i was gaining 33-37 lp. Now i played my first game in S2 and my Lp went to 25 all of a sudden is S4 so much lower in MMR that S2 or is it something else that caused that?
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