Data Master reads lower volts

I have a 94 z28 with 6 speed. When I scan my car with Data Master my ing. volts read 13.2-13.0 at idle. When I am running at the track it will read 12.8-12.6 volts. I was checking it out yesterday I had the datamaster hooked up and I had a auto meter volt guage hooked directly to the battery. The data master reads 13.2-13.0 volts and my autometer guage reads 13.8 volts as well as the farad cap I have hooked up for the stero. I have a brand new altenator and belt and just had Auto Zone check it out and every thing was fine. Why does the data master read lower volts.

Max, I have noticed this also, my DataMaster reads about .6 lower than the battery. The voltage displayed in DataMaster is from the 15a PCM fuse that feeds Ignition Voltage to the PCM. Out of the key it's the pink wires that feed about 7 fuses to the injectors, coil, auto transmission, cruse control, air bag, gauges and the PCM. I once thought of metering it to see if it was .6 below the battery but lost interest and figured the .6V drop was normal. Let me know if you dig deeper.