my watch can charge longer after reaching 100%

I'm guessing because the battery widgets are calling https://www.espruino.com/Reference#l_E_g­etBattery E.getBattery() which returns the battery voltage.

However, li-ion charging works in a two-phase process: constant current, and then constant voltage. It will try to start charging the battery until it reaches 4.2v and then stay charging at 4.2v while the charge current trickles down.

This is probably the reason why the watch may say 100% (which means the battery is at 4.2v) but can still continue charging as the second phase of the charge cycle isn't done yet.