This gets said every year but it bears repeating: ship dates are not necessarily related to sales figures. In order to make that correlation you would have to have manufacturing data (how many phones does Apple have ready to sell on launch day, etc).
And it's not like they stopped making phones the day of the event either.... still pumping out somewhere around 500k phones per day if past stats are still accurate.
So between preorder night and ship day on the 20th they can pump out another 3.5 million orders.