Broken fares. Orbitz is combining two separate fares into one ticket. This has been around for ages. BWI is a WN stronghold, so it tends to have cheaper fares. FLL is LCC city. Back in the NW days, I booked DTW-FLL-IND-PHL and return for less than the DTW-PHL non-stop price being charged by US/NW. The airline websites generally will not price out broken fare combinations. Matrix is probably the best at this. As you can see Google flights did not actually find the cheapest option. You used to be able price these out with multi-city on DL, but it's been broke for awhile now and often does not work. You can just price on Google or Kayak which will pull them up on delta.com.