I'm leaning towards the wiring as well. You're talking about fractions of a 1 volt making the gauge react. Any excessive voltage drop will cause the gauge to read leaner. That can happen if the signal wire is too heavy, the ground is not just right or solder connections are huge and lumpy. If you trust the wideband readings, try to dim the hell out of the narrowband display, ignore it, or get rid of it.
That's about all I can think of.....
Jay