Older electrical meters weren't all that accurate, especially for larger systems - that's true even today. They're analog as an only option of measurement due to incredible amounts of power going through them (too noisy for hall effect sensors). Large buildings are reactive loads to the grid, almost like an inductor. Anyhow, older meters picked up velocity for usage calculations.

Some A/C controllers carefully modeled how fast the power meters accelerated as an instrument, then used that information against them, throttling and ramping power use to slow the measurements down, basically causing under-metering.

Saved USPS millions a year in power bills.