I'll have a chat wi my colleague Brin who's a bit of an expert on smart meters as we work with some of the manufacturers. Seems from this page it's an issue with calibration changing over time, as with lots of measuring instruments, so I guess energy suppliers and also the network companies want to know more accurately how much energy is being used. Fully agree it's not a helpful way of communicating and the suppliers are digging themselves a hole, again!
https://www.gov.uk/guidance/electric...-certification