I see a lot of ball lightening builds praise Elementalist aspect and it doesnt make sense why

Core or Mastery Skills cast at or above 100 Mana gain a [20.0 - 40.0%] increased Critical Strike chance

I am sure I am wrong but if your first mastery skill cast with ball lightening is say at 100 then the additional casts will most likely be below 100 as you mash the buttons to get balls orbiting around you and your mana is bouncing around up and down depending on replenishment. To me each orbiting ball is a separate skill cast. So.. how does this aspect apply then? only to the "first ball" cast since that was at 100 and the rest of the ball cast are most likely under 100 mana. So does crit chance only apply to the first ball?

Edit And I understand that you can have more than 100 mana but the general premise of this still applies.