Battery drops from 85% to 83% instead of 84%. Why?

[The Penguin]

I just got a POCO F3 five days ago and I use the Accubattery app to notifiy me when the battery reaches 85%. However, I've noticed today while I was using the phone that the battery dropped from 85 to 83% instead of 84% but after that it drops normally. Anyone knows why?