Re: Apple Watch is not accurately calculating my mileage on runs.
Welcome to iMore!
My fiance' did extensive testing of several different fitness trackers, all at the same time so he could figure out witch one he wanted. Even with my Apple watch it was always extremely off compared to the others. Specifically counting steps, the Garmin (I can't recall which one) was closest so that's the one he bought. Him and I both have about the same stride length and he always does anywhere between 2 to 3k more steps than me when we go shopping all day on the weekends.
When I installed iOS 10 beta 1 on my 6s is when my steps finally were the same as his, so my daily steps went from 600 to 1k a day to 3k just walking around the house (we have 3 floors). As a test he walked out to the mailbox and back which was 57 steps (he counted) and his and mine were only 2 to 3 steps off.
Now with iOS 10 Beta 2, Apple broke the crap out of it again. This was apparent in the extreme after comparing our step count after going through the Seattle airport and then Chicago O'Hare. I use the default activity tracker.