iPad Pro Battery Issue

In preparation for a long flight, I charged my battery to 100% and completely turned off my iPad Pro. I just boarded the flight, turned it on at its at 89%. This is not the first time this happened. But now I have confirmed that it’s not my brain playing tricks on me.

I also put it into airplane mode and made sure WiFi, Verizon service and blue tooth were off before turning off.

Any guidance would be appreciated. Thank you.