That is because you are probably pretty close to the flight path. I.e: right after take off or you just reached a waypoint, so you are very close to the desired FP. Try this: disengage autopilot, fly a couple of minutes perpendicular to the flight path (about 90 degree left or right) You will see on the Abris and on the HSI you are off of the flight path. Than you can engage route mode, and try what is the difference between DTA and DH.