The MMR system ?

OK so i posted already once but this time it's even worse. So my mmr was around normal, op.gg said it was normal but also i was geting around 20 lp per winn and losing around 18 lp per loss. With that in mind, i won around 8-10 games in row making it from plat 1 to Dia 5 + i won ANOTHER game while dia 5 and got only 18 LP. At that point my mmr was dia 4 and finaly i lost a game, lost 19 lp. I though it was misscalculation. Now i was constantly winning and losing a game in row and i've been geting only 15-16 LP per winn and 19LP per loss. Is that realy ok for dia division ? Also yeah, the moment i lost 1 game after my 10 game winning streak, op.gg said my mmr is "lower then expected" and since i was geting 15 lp per winn it was obviusly somehow true. Is that normal for low Dia elo ?