Why your iPhone battery meter sometimes goes haywire

I understand alternate facts. Sometimes my iPhone’s battery meter delivers them.

Fact: I have a 18-month-old Apple iPhone 6. I keep it in god shape, never dropping it, subjecting it to extreme temperatures, or dunking it in bodies of water. I charge it regularly. 

Fact: On a recent weekday afternoon, I picked up my iPhone, which had been sitting on my desk untouched for hours, and noticed it was reporting just 18% battery life. It was officially in the red. I quickly plugged it into a charger and, a moment later, the rating was 43%.