Have you looked into why searching ATL-FLL as a one way gives you the lowest price economy (currently K fare), and searching round trip with DL outbound and B6 return is putting you in fully refundable business class? Is there something on your end you are doing or is your travel engine forcing it into C class?
Are you searching for a refundable ticket on the return on B6, that might be causing the entire trip to price as refundable?