it's not clear what you're expecting and what is wrong... Could your please explain?
In general I see some issues with divider:
1) Two 20 k resistors leads to 40k continuous load on the battery. So, the battery will have leak and continuous discharge.
4.2V / 40k = 105 uA. This is too much for power off mode.
2) With divider you're also needs to take into account VBAT input impedance. I don't know exact value, needs to check the datasheet, but as I remember, ADC has about 10k input. It needs to be taken into account
3) The divider leads to voltage divide (not offset like diode), it needs correction multiplier in the firmware. Current firmware don't have correction multiplier, just correction offset, because it expects simple diode instead of divider on VBAT input.
Battery correction multiplier needs additional calibration value. So, it needs to setup vbat_offset and vbat_scale. It's going to be a bit complicated for ordinary user. I think it's better to keep things simple and easy.
