Huh? Throughly confused by your title and question. Title says "won't start shipping until the 20th". Message inside says "arrive on the 18th." What am I missing here?
Basic shipping logistics:
Customer orders
1- Item gets shipping label
2- Put on a shipping pallet
3- FedEx picks up pallet and puts on plane
4- FedEx waits for plane to fill up with pallets then flies to AK (customs check) then mainland U.S. (typically CA, IN, or TN)
5- Fedex separates pallets and puts packages on planes (or truck if already as destination city) for final destination city & delivery
Apple has to over estimate delivery time because their contract delivery time frame w/ FedEx for deliveries from China is one week after it picks up the pallet. So if it takes Apple 5-10 days for your iPad to roll off the assembly line it has to add another 5 days for delivery because it has to assume the longest time frame. But delivery companies like to delivery as fast as possible which is why most people get their iPad before the last promised date.