MMR - Match Making Rating, hidden rating so to speak. Can be over or under the current elo you are.
I believe 1300 Elo is Silver 3 (I am at 1400 as Silver 2 atleast) and Gold 1 is 1500, so not 100% correct here. But you said you where Silver 1, so lets say you had about 1450 Elo.
In your promo you lost to two teams that had an average Elo that was lower than yours, so you lost a lot of Elo because of bad match making. Then you won your 3 Faker games, but that was against people that was lower Elo than yours, again due to poor match making system, and you didn't gain that much Elo. This is the same as when you are playing ranked matches (after promo) and you gain 2 rating when you win, but lose like 20, this is when your Elo is higher than the Elo of the place the game
thinks you belong. The same thing goes on in the placement matches, you just dont see the numbers. In the end the game sums it all up and place you where you belong.
Then it is your job to show the game and the system that you are better than what those 10 games "proved" and start to climb. Everyone falls down after placement matches, I fell from Gold 4 to Silver 3.
Now if the system is stupid is a whole other discussion..
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You can check your MMR at op.gg or lolking I believe (not sure if Lolking got mmr or just the elo from the ladder system, but pretty sure op.gg got mmr aswell, I cant check mine as I havent played a ranked game in a month or two so not enough data.)
Felt this video explained it well:
https://www.youtube.com/watch?v=YQnEcyEctgc