This has been knocked about on each cars' forum.

Bottom line: the dash display (does FEH have one?) and your manual calculation use the odometer for distance. The navi uses GPS. Odometer is actual distance travelled (assuming its calibrated correctly); GPS is interpolated from samples over time, and the dots are connected by the computer...with varying results. Any dropouts in satellite signal, and it has to guess or dead reckon.

Each position fix can be off by up to 10 meters...compound that, especially if you're changing direction often. In reality almost all of those errors balance out. But not always, as you've found.

I've also read that some map+GPS navi systems use the calculated distance of the roads you drive on. That distance is literally measured down the middle of the road, which you are not on, especially as you turn. That difference could add up, depending on what routes you drive.

Personally, I prefer the odometer-based display. It correlates more closely with my calculated mpg, even if its not as rosy of a picture.