Voltmeter low reading issue

1966 Barracuda, 318, low reading voltmeter, Amp meter in dash showing good charge, maximum charge on battery 13.2, under dash voltmeter max read 11.5 volts, which I checked with auxiliary voltmeter, also with headlights and vent fan on high dash amp Gage shows charging, but both voltmeters drop to 10.5 volts, what gives???