Can someone explain MMR to me?

I understand if the games hidden MMR for me is lower than my current division, it will give me less LP per win and more LP lost when i lose. The problem im having is that ive been doing somewhat decently. My win-loss ratio is 41-32, so im at a 56.2% win rate. I was placed in Silver 3 after my promos and ive climbed up to my promos into gold 4, only need one more win to enter gold 4. I know it might not be my true MMR but op.gg is saying my MMR is at a high silver 2 or low silver 1 which correlates to my LP gains and losses, ive been getting 15-17 LP per win and -20 LP or more when i lose. Why does the game think my MMR is less than where i am at despite making it though multiple promos and have a positive win ratio? I can understand the game thinking im close to where im supposed to be with my win rate, but 3-4 tiers lower? Whats going on here?