It's also realistically possible. If the battery is getting some crystallization a high amp charge can break that down. Not saying that's what happened here (probably software inaccuracy) but it's a real thing.
A few factors can be part of it, between the software still being in beta, and it all being an estimate based on various readings related to the chemistry of the battery which can also be a little different at times. I went from 85% to 84% then to 85% again and down to 83%, and after installing iOS 11.4 it's showing 88% now.