Due to the individualized average, information about the actual distribution of water use is obscured. That could be between household members, or households vs. businesses. The unit of observation is the water billing unit, but the unit of analytical claim is one average German inhibitant. There could be other analytical claims calculated from the water billing units, e.g. regional or neighborhood-based water use. Or there could be other observation units than the billing units / households, e.g. smart meters at every single tap, to allow for other analytical claims.
The way how the categories are forged, renders information invisible that might have been valuable for political debates. When drinking qulity water is used to clean cars, this can be problematized, however the combination of practices into categories (however necessery) hampers to state how much water is used in particular practices. The data here can only be read in a mingle of watering a garden, providing water to animals, washing a car or the living room, which might be assigned different importances and values in political negotiations on water spendings.