This gets a bit complex...
But a quick and easy summary would be:
"How does the system actually know how much time or life is left in a battery?"
The answer is not as simple as a Voltmeter or a clock. Instead, an algorithm must measure electron flow and then estimate the variables.
Let's say you put a AA battery into a wall clock. Given the initial charge and voltage and the steady use /droppage, this would be an easy estimate.
But on a notebook computer, it is nowhere near steady and can vary wildly from within minutes, to within seconds to within milliseconds.
When you have some idea how it works, you are amazed it works at all.
The best way to calibrate a new battery, especially if you have installed a new or different Operating System, is to charge it as high as it will go, then run it until it dies. Do not charge in between, just run it until its dead. Then charge up to full again. Usually about three repeats of this is enough to give the software a good range to start its estimations with.
That sounds tedious, but with only a little planning, is often easier than you might think to carry out.