Why is the distance reported in my Health App on an iPhone 6 20% too high?

I have an iPhone 6 that I carry in my pocket. When I do fitness walking I usually use the Strava App on the same phone. I also have a Garmin Forerunner 610 that I sometime wear. I got suspicious about the distances my phone was reporting when I went on a measured 10 mile hike and the iPhone Health App showed that I walked 12 miles. I have since used Strava, and my Forerunner, and looked at the health app for a series of walks, and the Health app always reports ~20-25% more distance then I have actually walked. Strava and the Forerunner report almost identical distance (I think the variations are due to using autopause) Why doesn't the Health App calibrate to GPS? Strava is accurately recording my distances on the iPhone 6, so why is the Health App so far off?

A friend suggests that it might be the jingling around in my shorts pocket when I walk, so I may try putting it on a belt clip for a few walks, but I thought the iPhone would integrate GPS data into it's data collection, not just use the accelerometer.