Just Fred wrote:
It's probably a "memory effect." Your camera probably had its last battery reading in memory and that value showed when you started the camera up. After it updated by reading the battery, the latest reading (0%) was given.
Computers have settings they retain even if the battery fails. Cameras these days are just another form of computer.
Bingo.
The last reading was in memory.
Batteries discharge slowly over time, and some functions always are a drain such as a clock.