This may be a little too geeky for a consumer device but it may be however the unit computes declination. It may also help to set it on level ground as this helps with triangulation by giving a static position equable to actual elevation. I spent a year or so dealing with some equipment dependent on GPS navigation and barometric sensors that was about as techy as civilians can take possession of. Lots more math and detailed setup than I imagine Garmin asks of its consumers.
Being near an airport or other larger equipment trying to do the same task is the first thing to look into as being at sea level you are nearly at the furthest point from space you will realistically be on earth. This limits the amount of satellites and complicates telemetry.
Also deposits of metal in the area can be problematic during occasional parts of the earths rotation.
See if garmin lets you into some advanced function that allows input of or shows a value for magnetic declination and check it against this