No surprise here, since just the voltage is measured... you would have to remove the charging power and measure again... a 'realistic' measurement is much more complicated (current that flows over time - coulombmeter for high precision metering).
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
No surprise here, since just the voltage is measured... you would have to remove the charging power and measure again... a 'realistic' measurement is much more complicated (current that flows over time - coulombmeter for high precision metering).