If it is considered that the drive train is connected during this testing, there may be differences between the freehub ratcheting mechanisms that cause the rotating differences you are seeing.
I recently built a wheel with an unused Ritchey hub whose freehub mechanism was fairly stiff compared to that of the previous hub. The only comparison in performance was how fast I coasted down even a shallow grade compared to my riding companions whom I had always outcoasted. I noted no difference with the new hub............