Once again, here is a map taken from Vol. 2 of Edward A. Freeman’s History of the Norman Conquest of England. (Click twice to bring map to full size.)  According to the author, as the early ecclesiastics converted the Kings and people to Christianity, each kingdom (or principality) formed a new diocese that was given the name of the tribe, rather than the name of a city (as on the Continent). There were a few exceptions; the Bishops of York, London, and Rochester were named after their city. Apparently the  boundaries of the Bishopricks were quite fluid, especially when someone died and the dioceses were redistributed. I believe this map represents the state of affairs around 1046 or so, after the death of Bishop Lyfing (who went to Rome with Canute) when the Bishopricks of Devonshire and Cornwall were combined to become Exeter.