I had the same issue on 2 different watches (1 Apple Watch Sport, 1 Apple Watch) using 2 different iPhones (5s and 6).
What I saw after several tests is, that if I carry my iPhone with me and the iPhone sits in a waist belt (usually in a horizontal position) either in front or in the back of your body, the Workout app would come out around 20% short with the Outdoor Runing profile.
However, if I carry the iPhone in my Hand or in an arm pocket on the same arm as the watch (in vertical position), the distance would be measured correctly and very precise.
Can anyone try and confirm this? Where do you carry your iPhone during your workouts and is it carried horizontally or vertically?
Maybe we can further narrow this down if others can reproduce this as well.
Calibration did not help with this at all. The distance is also spot on while cycling or walking. Just when running with the iPhone carried around the waist in horizontal position the Workout app seems to be way off on distance.
Also reported this to Apple Feedback.