When Apple announced its battery replacement program, it only expected to replace roughly one to two million batteries, according to well-connected Apple insider John Gruber. However, Apple replaced 11 million iPhone batteries during the length of its program – nearly ten times more than the company expected.

Apple’s battery replacement program began last January and ended December 31, 2018, allowing owners of the iPhone 6 and newer models to replace their battery for just $29. The program launched after it was discovered that Apple was slowing the performance of devices with aging batteries and were more prone to shutting off without warning. By offering battery replacements, it helped immensely on getting iPhones back to peak performance yet again.

Its huge sales in battery replacement is most likely why the company missed its shipment estimates in its fiscal fourth quarter, Apple said. It was also why it will ship fewer than expected in the first quarter. Apple revised its revenue forecast for its fiscal first quarter on January 2, as CNBC reports.

Cook claims the lower-than-expected iPhone sales and lowered guidance are also due to trade tensions with China, in addition to economic weakness. “In addition, these and other factors resulted in fewer iPhone upgrades than we had anticipated,” Cook explained.

Cook also felt as though some customers were taking advantage of the iPhone battery replacement program, saying “some customers [were] taking advantage of significantly reduced pricing for iPhone battery replacements.”  However, he never mentioned Apple expected nearly 10 million fewer customers to upgrade their batteries. Meanwhile, Apple did not respond to requests for comments.