The coefficients for the 6th order polynomial were generated based upon the measured discharge characteristics of a set of Energizer alkaline batteries. I had no expectation that the Duracells would track exactly -- simply because of what I am sure are differences in the manufacturing process (even the chemistry might be subtly different).If the theoretical is a straight line, it may be tough to justify the significance of any additional terms. They might help fit existing data but not really have much predictive value.
I am not surprised by the overall result. The only surprise was those few hours where the terminal voltage did not change at all. It's possible that the the electrolyte in the Duracells is more mobile than that in the Energizers, at least at that point in the discharge curve.
I am interested enough to repeat the experiment to see if it holds for another set of Duracells. But I cannot do that till I again have some free time -- perhaps in the next few weeks.
This is not an important problem. More of an intellectual curiosity.