Originally Posted by
BlueSkies
Uneducated guess, but I think because it's a 'swap' PCS looks at it differently than if you were trying to do a straight drop of one trip. Maybe Cbrezzy can elaborate.
This. It rarely works out for a shorter trip because the -xx is cumulative. 5 day with -15 total can't be traded for a 4 day with -14 total even though a single day may be -10. SRH under PCS/Swapping Trips/X-Day Move Formula.