The number could've been 15 million, but that would have had serious implications:
- Apple would have to buy additional production capacity. This will cost a whole lot of money since the capacity is only required to cover the peak demand. This production capacity will be useless in two months. Production halls full of expensive machinery will come to a halt.
- Produce stock in advance. This also will cost a lot of money. Inventory is expensive, especially when you're talking about millions of iPhones.
So 13 million is probably the number the smart guys at Apple calculated to be the most economically viable sales number for the opening weekend. Anything lower = money left on the table, anything higher = also money left on the table.