Just before I saw your post, I had figured this out. What I equated to being running on a flat open surface as being the solution to the inaccuracies, it actually was where/how I carried my iPhone. Just happened that when I tried it out on an open track and got the most accurate (to the meter) results I was hand carrying my phone in a vertical position. Did both a walk and run (separately) and each were very accurate.
The next day, I went running back to my regular route and again I was off 10-20%. Difference? I had my phone in a waist pack horizontal against the small of my back. Was about to take back the watch/phone to see if phone was a problem when I tried halfway through the run to stop and start a new run with my phone in hand. Guess what? Back to spot-on accuracy for the run.
So, looks like you're on to something. Seems weird that the phone has to be in some certain orientation to receive GPS signals accurately but at least I found I could get accurate results.
Obviously, this thing is not going to be as good at distance accuracy as a dedicated GPS fitness watch which is designed specifically for that. Although, I have to believe a 2nd or 3rd generation Apple Watch will have its own GPS. Oh, the downside of being an early adopter. BUT, I still love my watch. Overall, it is an amazing piece of technology just disappointing in this area right now.