Apple Watch: Workout app different distance than Nike Running (GPS) Is this a bug?

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Apple Watch: Workout app different distance than Nike Running (GPS)

I am using my Apple Watch paired with my iPhone 6 (iOS 8.4) for running. There is quite a difference in the distance tracked between the watche`s Workout app (running outdoor mode) and the Nike Running app (which on the phone is GPS based). I am wearing a belt on my hips to carry the phone, the watch on my arm. The distance is in average 10% less on the Workout app. I run up to 10 kilometers. GPS availability and network are always good (no high buildings, etc).
Is this is a bug? Why does the Workout app not use the GPS data? How can this be fixed?
Thanks!