Battery EMF vs SOC (state of charge)
While the equation you cited is indeed the basic reaction by which an alkaline cell works, the actual picture in a commercial cell is much more complicated. As the cell discharges, other reactions come into play that compete with the original reaction, and the external terminal voltage you can measure is a complex function of these various reactions combined with the effects of increasing internal current leakage and rising internal resistance as the concentration and physical distribution of reactants changes.