My Garmin Descent does not accurately record outdoor run distance

I have compared several runs now with my iPhone in an armband to the Garmin descent MK1 runs and the distance are consistently different.  The Descent MK1 always measures less than my actual run recorded on the iPhone or just running around a track.  Are there any fixes for you run software, algorithm , or GPS recording so the watch will correctly record my run distance.  I am running in Florida with good GPS coverage and away from any city areas.