AoE has a 20-target cap, and even with all the ball lightning that spawns, there are not enough adds that spawn to hit that cap.
If you swap back and forth from:
(Only damage done to unharnessed in the last phase.)
(Only damage done to ball lightning in last phase)
Any damage that you did to unharnessed while aoeing the ball lightning is all extra damage that would not be there otherwise, because you would not be doing anything else there. For the most part you only hit the unharnessed when the ball lightning was up, so my 16 million estimate was a tad over, but 12-13 million is still 7-8% of your overall damage.