I knew it used arm swings when not tethered to the Watch, but it has always been very accurate for me when I run with my phone. To the point, I never bothered to see if there was any differences between the two. Do you start a separate running App on your phone to measure your runs, while wearing your Watch for comparison purposes?
Yes. My preferred running app is iSmoothRun. It uploads my date to Nike, Strava and RunKeeper, but before I discovered it, I would have all 3 apps open on my iphone.
When I start off, I start iSmoothRun on my phone. This gives me the iPhone's GPS data, and provides audio feedback over my headphones as to pace for the last 1/4 mile, and overall average pace, steps per minute, and it used to do my heart rate, but I now use Garmin's heart rate strap, which doesn't broadcast the data in a way the iPhone can receive.
I also start the native workout app on my AW (s0 running OS 3.1), and select outdoor run. iSmoothRun can show data on the watch, but I have Apple's work out app running instead.
I now also start my Garmin 735XT. I have the 'run' heart rate strap, which provides further metrics. I confess, I am a bit of a data junkie.
I noticed the distance discrepancy between the iPhone's and the Apple Watch's as soon as I began using it. I assumed it would improve after calibration, but the discrepancy remained. At first I was highly critical of the Apple Watch because of this. Not least because it made it seem that I was running slower than the iPhone had led me to believe. BUT after a series of half-marathons that my iphone consistently thought were 14 miles or more (rather than 13.1) I began to doubt my iPhone, and better appreciate my Apple Watch more. It's assessment of the distance was proving to be more accurate.
My Garmin is my most recent purchase. It's a dedicated device for the single purpose of measuring running, cycling (and swimming, although I'm not swimming with it at the moment). My friend has one, and it has accurately measured some of the half's we've done. It's leading me to believe that Apple Watch software is more accurate than I first thought.