You're duplicating the coefficient, it says it was increased by +100%, not doubled. By the way I did try your way and got a lower damage than it is on the NPC. But didn't you say that damage here is a lot higher than it was on retail? Maybe doubling it makes it a lot higher than it was on retail. Currently I'm doing some research regarding it.
I might be wrong though.
There is not much we can do, to be honest, all that we are doing are theories, specially because most of these formulas aren't actually in the internet, and that +100% can be interpretated in both ways: yours, where you duplicated, and mine, which I added +100% instead of duplicating.
I think that all that we can do for now is wait for an administrator.