The D120 isn't wrong. The voltage at the D120 connector isn't the same voltage as what is at your cigarette lighter. Because every wire has resistance, voltage drops occur all over an airplane. You can't calibrate this out because it changes over load. The best thing you can do is run the D120 directly off the battery without sharing it's power wire with any other devices.
Also, if you use Bob Nuckol's system with a standby bus powered by a diode, your essential stuff is a diode drop lower (~0.4 to 0.7V lower).
Overall, absolute voltage is not as important as changes. If you know 13.4 is where the plane flies with everything working fine, then suddenly having 14V or 12V in flight is when you worry.
