Obviously I wouldn't read too much into this as it's likely a tooltip bug, however it would be interesting if they tweaked the mastery to give different bonuses based on weapons much like the old warrior weapons mastery talents did.
99% chance it's a change to fix the animation bugs with non-bows. Likely they're splitting the proc (not the mastery passive, but the actual proc spell -- which is what this datamined bit is) into different spells for the different ranged animations.
its true that this is easy to calculate out and all but, i dont really see why hunters dont have this i mean look at mages they got pure damage increase modifier based on mastery depending on their specc, and i belive there are other classes out there that has the same, and to be frank those masteries are alot higher then hunters so
Ghostcrawler said this on Twitter
@Un1v3s4l_Gaming Just a bug fix to make the effect use the correct weapon.
Could be a fix to stop wild quiver from making gun sounds if you have a gun transmogged to a bow.