I believe cars are *usually* assigned as they come off the production line and are scheduled for shipment. However, once that happens, it's possible for the order to be canceled. If an order cancellation happens while the vehicle is already in the shipment process, it is probably matched to the next order in line at the destination it is already planned for. So in that case, it's possible for someone to "jump the line" and get a VIN with a very fast delivery date. If no matching orders are found, then the car becomes available in the general inventory of that Tesla location.