Voltage variation

I still haven't had time to work with my A&P to remove my (newly)
flakey JPI EDM-700 unit. (As you may recall, some of the LED segments
that make up the number display are going in and out.)

Someone here suggested that it might be due to low voltage, possibly
indicating an alternator problem. I've started watching the voltage,
and have noticed something odd:

The JPI has its own built-in voltage indicator. It will usually read
13.4 or 13.5 volts. We also have a stand-alone, panel-mounted digital
voltage/amperage meter, mounted a few inches below the JPI. It
usually reads a few tenths of a volt different than the JPI. (I can't
remember if it's up or down...)

a) Is this normal? Why would they read different voltages?

b) What voltage is normal? Is 13.4 too low?

Thanks!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"