Apple has announced and shipped its new high-end iPhones in September since the iPhone 5 in 2012. This helped to contribute to a 4% quarter to quarter decline in iPhone unit sales when they are adjusted for inventory changes. The iPhone 7 still had a 3% quarter to quarter decline in unit sales after inventory adjustments even with 9 selling days in the quarter. This would give the company 9 selling days for the new iPhones. Understanding the number of selling days will be a clue to this.

Source: Forbes August 30, 2017 18:11 UTC