The release from Microsoft does not say that sales will be low in March.
"We definitely expect we will trail in February as a result of our supply constraints. By April, we will be in a very healthy inventory situation."
Says they were constrained in February.
Says By April inventory will be very healthy. That means during March inventory will build. This I suspect is to they can have a very large (very healthy) inventory for price drop and GTA4. So no problem finding them in March.
Its libraries that sell systems not a single game.