Why combine AP flour and bread flour?
It could be as simple as a bare pantry shelf and an adventurous cook who combined them when there wasn't enough bread flour for her recipe. King Arthur Flour went above and beyond! Testing loaf after loaf to compare crust, crumb, and texture with a variety of flours. Are there differences? Certainly, but perhaps not as dramatic as we might think. 

How to Substitute Bread Flour for All-Purpose Flour