Semi related and somewhat humorous...
Retrofitted a Garmin 695 to our RV7 a while ago, all was well and it is a super piece of kit.
A short while later, my buddy said, "why is the standby compass 30° out ?"
I looked, we had a fiddle and couldn't fix it. It is a Falc*n gauge, so we put it down to remote hacking ;-)
Then, I was working on our RV8 and installing the similar unit. I placed the GPS aerial close to something metallic and it jumped over - yep, the GPS puck aerial has a magnetic base
So, our little whiskey compass is just looking at the GPS aerial.
Some things are easy to figure, some take a coincidence