Voltage on VOUT of voltage divider not meeting calculation
Here are the things to check. You don't have to do it in this order.

First, use a voltmeter to measure voltage at the ADC input. If the ADC has error, you should characterize it first so you can calibrate it out.

Second, check the divider input and output voltages with a voltmeter. Use the divider equation and compare with the measured values. Does Vout = Vin * R1/(R1+R2)? Allow a tolerance for the resistors (5% or 1% or whatever is appropriate for your circuit). Is the divider output higher than it is supposed to be? Then somehow, current is being injected into the divider output by the ADC or other circuitry. Maybe there is an internal pullup resistor or even an external one you didn't notice. If the measured outupt is lower than it is supposed to be then something is removing current from the divider output. This could be the ADC input itself. Sometimes the ADC may have an equivalent resistance of as little as 100k or so, depending on the ADC. Or there could be a pulldown.

If there is a pullup or pulldown, or large error introduced by the ADC current, you may opt to just calibrate it out rather than try to eliminate the error. But it is definitely a good idea to understand what is causing the error. One way to eliminate the error is to use smaller value resistors, but that has the drawback of increasing the battery current (even when the device is off, those resistors are draining the battery).

Hope that helps.