Apple also reports each iPhone / iPad shipment from China into inventory or the sales channel as a "sale" for PR purposes. This is especially important to them when iPhone sales traditionally slow down after Christmas.
One famous case of this was back in Jan 2008, when Apple reported "selling" 3.7 million iPhones in 2007.... yet AT&T only reported 2 million iPhone customers. There were about 300K sold overseas, so that left
1.4 million missing phones. (See also
The case of the 1.4 million missing iPhones - Fortune magazine, and many more such articles.)
Analysts later figured out that anywhere from a half million to a million were actually in warehouses.
This channel counting is now so well known, that someone usually brings up the topic on Apple quarterly calls to try to pin down actual end user sales per period.