Short short answer: because those voltages are "nominal", not what's really observed in-circuit. The available output charge (ampere*time -> charge) is much more controllable during manufacture, as it's directly related to the quantity of available reactants within the battery.

You forgot to add the heat units, like BTU and calories, and the units for food like "kalories", and the units for air conditioners, like "frigorie".

You are right, it is even more complicated, than the picture says.