Dex doesn't change the bow dmg as seen on the bow. It changes the dmg you do with that very bow. How else would you explain me criting a foe 35k+ with a bow rated at 156/268 dmg?
I'd like to close by quoting Bellegar's famous last rhyme:
"Cursed be the mother that bred thee! The wench must have been a hell-spawned banshee!"