There seem to me to be similar inconsistencies in bird names. For example we use sparrowhawk but honey buzzard. Chaucer used spar-hawk, Tennyson wrote about the sparhawk in Sir Launcelot and QueenGuinevere and Bewick made a woodcut of a sparrow-hawk.
We write stock dove but woodpigeon. (The BTO and RSPB both write woodpigeon but the Collins Guide and Wikipedia use wood pigeon.) We write blue tit but blackbird, although Bewick called it a black ouzel.
All authorities write shore lark but skylark
and shelduck but ruddy duck, etc.
All the finches are one word (gold, green, bull, haw, and so on)
but all the buntings are two (reed, snow, corn, etc).
I'm not sure it matters all that much and I expect most of the inconsistencies arise from common usage. If you want to read more about this subject you can see what the International Ornithological Committee has to say about it here. Even it doesn't have all the answers.
What's in a name? that which we call a rose
By any other name would smell as sweet.