Dispersed lightning arc.
Are you sure? did you test it during the campaign?
because I noticed it perfroming inconsistently during the campaign, but I believe that was because some of the necron light cruisers have left and right broadsides, not turrets.
I am actually not sure if the left/right 180 arc guns can be fired simultaneously, or if it's just one set that picks either the left or right arc to fire along, but if you have a ship to the left of you you, and then 3 ships to the right, it would mean that if you fire at the left ship, the guns should not arc to the guys on the right: because they are not in range and arc of the gun that actually fired. Just possibly the guns that didn't fire.
Also, the guns take forever to reload on necrons, so it takes a while to actually see the result.
I have definitely seen lightning arc hit multiple targets and hit a single target in campaign, but I have not yet seen it fail to spread when I was certain that it should have. Just a bunch of ambiguous situations where I wasn't sure.
If it was in campaign, check to make sure it's not your capital ship, which has the upgrade which allows for gauss weapons to have the same effect.
I played in multiplayer just after posting to check, using a carin battleship in a mosh pit of imperial ships- there wasn't any splash.
If it's only showing up for you in campaign (where you specifically start with the upgrade that effects the gauss whips, not the arcs, which is what the stance is supposed to effect) then I think that's telling.