Uh. What? The power tends to go to a young girl that they last thought of. Its vague enough as it is and is really hard to pin down - hence the whole reason why they're trying to get Pyrrha to do the transfer. There isn't any guaranteed "tendency" or direction of where maidens would be created. Also, take into account the fact that Maiden's are necessarily so shortlived. After 40 years of experience and many young girls to think of in your last moments, it is a very tricksy situation. So I don't see how it would be easy to keep them all in a single area.