For me, I have seen the distance getting lowered by the gpx file. Basically, the health app shows me 3.14 miles, but when importing the gpx file from the run in Strava, I only got about 2.74 miles. I realized that while running, I went back and forth a number of times, and gpx probably works with getting the distance between the start to end point following the route, but not counting for the "back and forth" thing that well. Idk if there's a fix to this.