This is not especially unreasonable. A good many early references place Arthur in Devon/Cornwall. The earliest probable reference to him, in the Gododdin (See Other Works), seems to place him in a Northern British literature, however, one should remember that Gereint (possibly the Devonian Arthurian character, though see Pearce*), who also appears in the poem, is there "for the South". This suggests that, if the poem is a real reflection of the group, the Gododdin warband was made up from those across the British nation and would have access to a broad range of cultural role-models. For a detailed discussion of the movement of the people in the genealogy, see Pearce**.