Fairly certain the damage inflation correlates with the Finisher effects. I decided to play around with it on my assassin for a little while (did not do much extensive testing sadly), but the higher hits (84-87mil in non-optimal heroic group) always happened with triple finisher. Said high hits also did indeed coincide with the mob's HP... roughly 20 million hits pre-50%, and 80 million hits exactly on the 50% mark and from there on.
This seemed to also be the case with Early Advantage, a proc I also wanted to test with Finishers. Had similar results w/ steady 14/60mil hits which seemed directly proportional to mob HP/wearing Finisher effects.