Yeah, that's what I suspect too. Maybe it's most accurate with the preamp disengaged. I'll do some testing with all my basses so I know what to expect.My guess is the resistance of the circuit as a whole due to plugging it in. It has to read the power though everything (pickups, pots, switches, etc). Just testing with a bare battery, there is nothing else in the circuit, only the tester and the battery.