Why does reading meter readings at checkout go wrong so often?

Reading meter readings at checkout often goes wrong because you measure at a time that suits you, not at the actual moment of departure, which means extra consumption is included. Moreover, you have to keep track of two readings per change without mixing them up, a meter reading provides no explanation for high consumption, and there is no standard to measure it against. Continuous metering per unit from arrival, as with Staywatt, makes consumption visible as it happens and thus prevents this kind of dispute afterwards.