Ah, cool:
An aside --
I've wondered if a GPS-tracking app running on the iPhone will get all the GPS data and not let it go to calibrating the Watch.
I say this because I've tried using two jogging apps simultaneously (Nike+ Run and Asics), and of the two, the only one that tracked my run was the app I started first. The second app sat in the background without gathering any data.
If the GPS data were shared among apps and the W at the same time, there's no reason for them to get anything other than an identical result. But if the data stream can't be used by more than one service or app, it would explain the differing distances.
I'll recalibrate my Watch and go for a few runs with my phone. Later, I'll take along my Garmin 410 for comparison's sake — if I get around to recharging it, anyway.
I remember someone posting here that when calibrating the watch, you shouldn't run any GPS tracking apps, because as you say, only one app can use GPS data at a time. I don't recall what their source of information was, though.