As I already told you. MMR doesnt increase at the same speed as you Rank does.
iE
You start at 2000MMR(P1). Now you win nonstop. Now you're D1. Problem Your MMR is still D3.
And you still get matched with people at D3. The system showing you you're D1 means nothing, it's just a visual gimmick that alters your LP gains/losses in a way that your visual rank roughly matches your actual rank.
And this system fails the second you do something not calculated:
Win-streaks, Loss-streaks, Promo-Dodges, DuoQing to some degree.
By decreasing your LP/Win, increasing your -LP/Loss the system tries to let your MMR catch Up with your Rank.
Your Rank progress gets slowed down - your MMR growth per win is unchanged (since it never changes).