How is the damage for Reverse Slash calculated?

I have a question regarding ESO skill reverse slash.

In the skill description it says: "Gives up to 300% more damage against enemies under 50%". The thing is, how is that up to computed? Is it random?

I tested against some random npcs (hit them with the skill while under 50%). My base damage on skill is 2988 dmg (let's round to 3k). So 300% more dmg would be 3k + 3*3k = 12k. Or if my interpretation on "more dmg" is wrong would be just 3*3k = 9k.

The damage I've done was something like 7k, 4k, 5k, 5k, etc. (averages at about 5.5k overall). Now... that seems way low than expected (even if I factor in armor). The other execution I use (nb - killers blade) seems to do way more damage and all reliable (no variations) - but... that's magical damage and it doesn't say "up to" in the description.

So... to sum up the question is: What is the real damage the skill does? How is that "up to" computed? Any1 else tested and got different results?