Mystic Reverb is bugged so that if it kills, it triggers the damage twice (Why? Idk, but it's easy to replicate, especially with Geist and Lash)
Has no impact on the person it killed since they would have died even if it wasn't bugged, but it's awful for anyone around them.
(Normally this would make Reverb deal damage = The damage Exchange did, but Reverb is boosted by damage amp ect, which is why it dealt slightly more here)