PAP-FLL is a different market than PAP-ATL. Because Delta operates a large hub in ATL they can charge a premium for non-stops into and out of the hub; and people pay a premium to fly non-stop. Whereas flying PAP-FLL offers many options on AA, US, B6 - some with connections and some without.
Yes, and this might be implemented through married segment logic, with the cheap seats PAP-ATL only being offered to those who want to connect beyond ATL. My guess is that PAP-ATL-FLL is booking into a (much) lower fare class than what you would be required to get for just PAP-ATL.