Well, firstly, it's made from assumptions. The list does not state what those assumptions are, and therefore it's basically useless to us unless someone provides them. In other words, if you assume the target has a million HP and a million Armour/MR, Vayne is automatically number one. If you assume the target has a million HP and zero armour/MR, Kog'maw is automatically number one. So, you can basically ignore the list.
However, I can state with some certainty that it's completely misleading. In actual fact, Twitch will have the HIGHEST overall damage output in the game, provided you assume his ultimate hits five champion targets and no creeps (before the full reduction). If Twitch does not hit multiple targets, his damage output (i.e. single target damage output) will be considerably lower than Kog or Vayne (and I believe below some others too, but you'd need more figures and more assumptions to determine which).