If you are using OBD2 connection to your vehicle, the battery voltage is requested with ‘ATRV’ command. That returns the voltage as the OBD2 adapter sees it and request is never sent to the vehicle. This is done because its faster and on many vehicles ECU voltage is a custom PID.
But many cheap OBD2 adapters do not have voltage calibrated properly. And there may be an deviation on the reading. It is possible to calibrate your OBD2 adapter, but its somewhat complicated and not recommended for beginners.
How to calibrate the OBD2 battery voltage - Connectivity / OBD2 - RealDash Forum