In the weapon enchants thread it was described that some slow powers proc weapon enchants for higher values, which was creating some confusion. Lightning enchant respects these scalings (including the 0x scaling from Mirage clone at-wills), but the arcs do no respect this scaling at all. The arcs proc for the exact amount no matter how slow or fast (Mirage weapon at-wills) are.
This is not to be confused with scaling off power/buff/debuffs. This is the scaling related to the speed of the power. For example Daunting Light proccing lifedrinker for 3x the expected damage.