I think alot of it was choice of wood, since several seem to be flat grained. I know hickory very commonly had the rings violated, and I suspect elm can handle it too. Native Americans had no problem using an edge grained split, like on some Wampanoag style bows, and I recall that they would sometimes split a sapling, and use the outer growth ring as the belly, and the flat split side as the back. They likely discovered quickly, through trial and error, which species they could get away with carving, and which they couldn't. This would be why you don't see any carved osage bows, but find plenty of carved hickory bows. They didn't know of any "rules" to bowmaking, aside from the ones they discovered themselves.
