Skipping Divisions Explained?

So to keep this as short as possible ill say this. I skipped my promotions from gold to platinum completely. Once I hit 100LP I automatically got placed in platinum without doing promos. Once I was in p5 I won again but this time when i was 100 LP i still had to do my promo's. Can someone explain this to me? I'm not nagging just trying to understand. I get around +31 -6 if anyone is wondering.
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