I would expect that the ratchet mechanism in the road bike freehub has more friction associated with it than your mountain bike's freehub.

Your test of chain condition is unorthodox, though not without some reasonable basis. As the chain wears, one would expect additional friction caused by the rollers not lining up exactly with the teeth in the gears. However, this sort of measurement is fraught with error (how well calibrated is your backward 'flick'?) and will vary significantly between different freehub designs.

Better to use the 12 inch rule as described above.