[CA] Compass differential

[the wizard]

I recently installed the ACU-400 with EV-1 and p70s on my Beneteau. After completing the Dockside Wizard I performed the 270 turn as prescribed in the manual. We have been cruising for about two weeks. There is a consistent +15 error in the autopilot [AUTO and HDG] on a variety of courses as compared to the boats compass and the Garmin Chart Plotter which are generally within 2 to 3 degrees of each other. For example the Autopilot reads 065, the compass reads 048 and chart plotter 051. The setting for boat type is Sail. All of the other settings are the default 12. Any suggestions as to why this is occurring and action to improve accuracy of the Autopilot? Thank you.