Hey,
There is a bug with battery indicator in Forerunner 245 Music (don't know if that exist on other devices). There is always a bug, when battery drop by 1% the watchface indicator shows this difference earlier than a system indicator. So for example watchface shows 88% when in system settings there is 89%. After some time amount is the same on both indicators. But later there is again - 87% on watchface and in system there is 88%.