Why combine AP flour and bread flour? I've seen some recipes that call for AP flour and bread flour. What's the point of that?

Assume AP flour has 10.5% protein and bread flour has 11.5% protein. If a recipe calls for equal amounts of AP flour and bread flour, you'll end up with a mixture that is 11% protein. Why not just find 11% protein flour in the first place?

Also, since the protein amount of flour vastly differs by brand, region, etc. -- how can a recipe make any assumptions about the final protein amount in the flour mixture.
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