@Riot Why is my LP gain so low ?

I have a 65% win rate in diamond 5 and I just got +18 after winning promos. What ? Last year I was getting +18 with a 45% win rate lol when I was tilted. Why am I getting +18 with a 65% win rate ? I don't get it. I'm getting even lower LP than I had in plat 1 and I just won promos and another game. Doesn't make sense.