DL 65 is the year-round FCO-ATL with DL63 the add for seasonal two-a-day service that starts May 31 and ends Oct 1 as you mentioned. But it must have initially been loaded for later in the season. DL Comfort+ is almost totally booked for 10/23 (1 middle seat remains) hopefully that is because they've proactively moved everyone (including you) from 63 to 65. Maybe the agent can see if you're also on 65? I would try to resolve sooner rather than later to avoid seat issues.