How accurate is op.gg?

i checked my mmr when i just won two ranked games and it went down by 50 lol.. i won another one and it stayed at the exact same number ;-; (note that i did not lose any ranked games in between). the standard mmr for my league and most other leagues seems to fluctuate by the hundreds when i check my mmr after every game too

does anyone know how accurate this site is because it just looks stoned af to me lol