When Virgin take over the JFK flights from Delta, I don't think we should overlook the issue of capacity provided by the different aircraft.
In summer, Delta's B763, which I think is normally the 226 seat version with 26 in Business, is being replaced by the 266 seat A333 of Virgin. (Is the latter slightly reduced after the Upper Class refits?).
In winter, Delta operated a mix of the B763 and 757, the latter with 168 seats according to seat guru and only 16 in Business.
The same applied when VS took over the ATL flights, the A333 replacing a B763.
The other factor is that VS are more premium heavy than DL.