Apple on Wednesday offered its most in-depth explanation yet for the current shortage of iPhones for sale,Cellular News reports.

“… Apple Chief Financial Officer Peter Oppenheimer said Apple had expected a sharper seasonal decline in iPhone sales, especially in the U.S. But demand, especially in the U.S., was much more intense than Apple expected.

Customers bought multiple iPhones at Apple stores, then unlocked them and shipped them overseas or offered them for sale online.

The current iPhone shortages, called “stock-outs” by Apple, point out Apple’s relative inexperience in the cell phone business. Apple introduced the iPhone, its first cell phone product, less than a year ago.

To date, Apple has sold about 5 million iPhones, according to various estimates.”