Not all of the pre-orders are delivered on the first day due to a limited supply. For example, I pre-ordered within the first 24 hours and my shipping time was one to two weeks.
Plus, I'm sure that the number being tossed around at the UPS branch could easily be incorrect. Just think about how often you've heard some hallway talk at work and the inaccuracies it contains.
64,000 would be a minor blip for UPS. Consider people who've said how many their driver had on the truck. 60-100 which means 600-1000 trucks. Anybody who thinks that few trucks could deliver the entire US is a bit naive.