Why are the battery volt limits for 0-100 between these 2 numbers?
Because analog input can take only up to 3.3v and battery is up to 4.2V so there must be some resistor divider to reduce voltage below 3.3V. Resistors were chosen 'randomly' by manufacturer for this design so no reason to multiply by 3.3. Across some fitness trackers I've seen the value varied a lot for each manufacturer/device model.
Was this code aimed for a more weared down battery?
individual resistor tolerancies can be even few % see e.g. https://www.electronics-tutorials.ws/resistor/res_2.html , can make some difference in the voltage divider for each device