The calculations of damage are done is a very particular way. It is easiest to think of it as an equation with 3 major components multiplied together.

Damage = A x B x C

A, is the basic damage of your bow as shown when you look at the inventory screen and any + Max/ Min damage from equipment or charms is added here.

B, includes all the bonusses to damage like Dexterity, From skills like Strafe or Guided arrow, A Might Merc, Pally Fanaticism aura and off weapon %ED and they are all added together (not multiplied by each other)

C, is where Critical Strike or Deadly strike is applied (=x2) and where the 0.75 penalty for Strafe and Multi-shot is applied.

So you see the two % your are thinking of are applied at different parts of the equation and hence have very different effects.