Things that come to mind. There is no gaurantee on prority mail delivery time. The first package was sent mostly over a weekend. During this time frame, you'll get postal lite from both government postal employees as well as contractors. Contractors, who transport mail from post office to post office, only work half schedules on the Sundays. Also, depending on final destination, your package will be routed to different sort facilities. Don't even try to figure out how the USPS system works...it'll make your head hurt.
Larry