The only distinction I can see is that we can't imagine how batchelor would be extended.
I don't think it's logically impossible to extend the definition of batchelor, just unlikely and hard to see why it would be done.
As cake covers such a wide variety of objects, it's easy to see it expanding as a definition.
( my other username is a porsche , posting shit pictures so you don't have to, Tue 23 Jun 2009, 22:15, archived)