[cptskippy]

As batteries discharge their voltage drops. This is known as a discharge curve. The curve is based on a constant current, different current draws have different curves.

http://learningrc.com/wp-content/uploads/2017/02/Lipo-Discha...

Battery capacity is guessed by the Voltage output at the current draw when measured.

Laptops usually throttle components, reducing current draw, when they fall below certain percentages. This prolongs the battery so that last 10% really does last longer.

This article goes into some nitty gritty details if you're curious.

http://learningrc.com/lipo-battery/

Determining actually battery capacity and runtime are educated guesses because all the factors impacting them are constantly changing.