Yeah, that's normal for my setup. The D10 is on my e-bus. My main bus voltage is normally 14.1 to 14.4 according to my AF-2500 monitor, and that's the voltage reading that I trust, confirmed with a digital voltmeter.
The e-bus voltage is usually about 0.7 volts lower than the main bus by virtue of the voltage drop across the diode. And the D-10 has always read on the low side by at least one volt. It's a quirk of the D10 that I'm aware of and live with -- to me, what's important with voltage readings are relative changes.
I have heard from various other D10 and D10A drivers that the voltages they display are almost always off in one direction or another. I don't know why this is. To me it's not critical and I haven't bugged Dynon about it. I'm sure somebody will.
)_( Dan
RV-7 N714D
http://www.rvproject.com