Paul,
I’ve seeen this behavior as well. Someone posted it might have to do with new iPad battery management feature.
I’ve noticed sometimes on my iPad 10.5, that I can’t get it charged to 100%, but if I unplug it around 95% still have about the same battery usage when I hit 75%.
I think it has more to do with difficulty of reporting state of charge based solely on battery voltage. 3V is fully discharged and 4.2V is fully charged (assuming normal 4.2V lithium chemistry). But the relationship to state of charge in between those two end points is not set in stone and subject to calibration issues. I'm wondering if they simply revised the voltage vs. state of charge lookup table.
Paul