I hope this isn't a dumb question, but I haven't flown a whole lot of GPS approaches and today I observed something that I cannot explain: There was a 2 degree split between the Desired Track (DTK) displayed on the GPS, and the inbound course shown on the approach plate.
If it matters, the approach was the RNAV 16 Y approach at KRNT, and I was using my club airplane's Garmin GTN 650. The database in the GPS was up to date, and my approach plate in Foreflight was current. DTK showed 159, and the plate showed 157.
Not a big difference, but they ought to be the same, right?
At first I thought it might be displaying track direct from my present position, but I had early turned the fix a little coming from the west, and was lined up a bit right which would have made the course needed to correct a few degrees less than 157.
And as I made minor course corrections back and forth across the final approach course the DTK never wavered.
Is there a good explanation for this difference that I'm just not aware of?