Battery meters are nonlinear (i.e. they don't follow a straight line) in the regions close to full and close to empty. Battery level meters also tend to go out

Battery meters are nonlinear (i.e. they don't follow a straight line) in the regions close to full and close to empty. Battery level meters also tend to go out of calibration over time. I think these two factors together probably explain what you saw (as you say, a battery physics issue essentially).