As stated above, the in-dash VM gives approximate readings at best. Just the position of your head while looking at it will make a difference. Years ago I made up a little longer voltmeter wiring setup to connect to the battery posts and the VM held in my pocket. Anytime I really need to check charging volts, I attach one end of VM wiring to the + and - terminals of the battery, plug the other end into the VM in a jacket or shirt pocket, take a ride and check it.
DPCM, USN (Retired)