Dexterity is main stat of a bard and equals its attack power. Attack power in character attributes is inrceased exactly by 15% when used it. Therefore my math is right.

I think you are confusing it with 'slashing resistance down' debuffs and such. For example Strorm's Eye does not increase your damage by 10% if resistance is down by 10%, you'll deal only 6-7% more damage.