I dunno, but I do know, just like in TBC, lightning overload only procs 1/3 of the amount the talent says it will if you only hit 1 target with chain lightning.
It has to do with the trajectory from when it leaves you to your target. It feels like it wants you to be square with your target (and square with the branch circuits) before it branches out to others.
Lul wut? One fun thing though - lightning overload does have a proc chance of 11% per target hit with chain lightning, so it can technically proc 3x on 1 3 target CL