Double Thrust is rather funny:
Deals 2.0 damage to an enemy 7 times :P
It doesn't even match what it says on the JP wiki, lel. But I looked further back and it seems that the original "Double Thrust" skill did 2x0.7 damage so I think that's the likely slip up here.
All the references I see to leveling up a skill only mention an increase in chance to happen.
The chance to happen is not the faulty description happening, it just means their skill has a chance to activate. The description of the damage modifier is still wrong ...