Heading sensor

[BosSH]

I recently installed a Raymarine fast heading sensor, which connects through a seatalk ng bus to our ES 78 Raymarine MFD. Our autopilot has a stand alone controller and fluxgate compass, so it is not interfaced with the MFD. I aligned the heading sensor with the binnacle compass and also with a hand-held compass, by using the compass offset feature of the MFD. Although the heading sensor often remains aligned with the binnacle compass, there are frequently times when it is up to 8 degrees off. Since the heading sensor is not interfaced with the autopilot, I have assumed the neither the linearization feature nor the alignment of the compass to the GPS feature is relevant to my system. I have also assumed that the compass lock is not relevant, although I have tried the lock in both the on and off positions, and it appears to make no difference. Any advice would be appreciated. Thank you.
Bob