RealDash uses ATRV command to read the battery voltage on OBD2 connections. A reply to this command is from the OBD2 adapter, the request is never sent to the vehicle.
Unfortunately some adapters are not calibrated correctly for the voltage, so they show completely different value than actual voltage of the vehicle.
Here is some instructions on how to calibrate your adapters voltage reading:
How to calibrate the OBD2 battery voltage - Connectivity / OBD2 - RealDash Forum