You're talking about this 5:59, right? It's because Demon Delta's ultimate is a 3-hit, and each hit is counted separately. So, assuming you can hit the damage cap on all three hits (the current damage cap is 399k) so its 399k x 3, that's how you get that total damage. This is really easy for Demon Delta in this case, since if you look closely, she had 3 Demon Power buffs and, on top of that, several damage-up buffs as well.