The ITA site was combining two separate one-way fares (one way on F6, the other on DL) as opposed to a single published round trip fare. And since F6 and Delta don't have interline ticketing agreements, there was no way for Expedia to ticket the round trip itinerary.

Since the total round trip fare was the sum of two one-ways, you could have purchased each leg separately through either Expedia, the respective airline websites, or any other Internet TA (such as Orbitz, Travelocity, etc).

ITA is great for building fare/itinerary combinations not found elsewhere, generally because they are simply combining two separate one way fares to present as a round trip "package".