The answer is obvious once I spent 3 mins with a spreadsheet -- aircraft utilization (and to a lesser extent crew turnaround, though that can be handled in other ways). The fine is hefy (about 50/pax for a full load) but presumably the profit from operating the day flight back is worth it for CX. Haven't checked the schedule, but is the day flight daily yet (it was alternate days originally)? Perhaps CX does the pre-6am arrival only on the day flight days (more likely they view schedule consistency as more important and just eat the cost even when they get no benefit from the early arrival).