I'm thinking there's more current from the more dense wiring section. The greater number of smaller wires in the same space should generate more magnetism than the fewer large wires in the same space. The current is transmitted on the exterior of the wire, so greater surface area of the "many wires" should mean more current and more magnetism, as a result. That would be my orientation, which might not be completely correct. Not sure why the thickness of the retainers would be significant?
CBODY67