The dual September unveiling of the iPhone 8 and iPhone X, but staggered release dates, was borne out of necessity and not “marketing,” Apple CEO Tim Cook told CNBC after reporting blowout quarterly earnings.

“If we could do anything we wanted to, we would have obviously shipped 8, 8 Plus and X on the same day,” Cook said Thursday evening in an off-camera interview. “But we felt like the most important thing for us to do was to announce them on the same day so we wouldn’t have customers buy an 8 and then we announce the X and ship it and they go, ‘Oh, you hosed me, I would’ve bought a X.'”

“That’s the reason that we did the stagger,” Cook said. “It wasn’t a marketing thing. It was we weren’t ready to ship; we were still working on [the X]. We accelerated the date at which we were initially planning to do iPhone X. And that’s how we got here and we’re going to learn a lot.”

The iPhone 8 and the redesigned 10th anniversary iPhone X were both unveiled at Apple’s big Sept. 12 event. But the iPhone 8 and 8 Plus were shipped 10 days after the event. The $1000 iPhone X, with no home button and an edge-to-edge super high-resolution screen, was not available until Friday.

Cook also told CNBC Thursday that the iPhone X was “quickly backlogged” due to strong demand. Asked when supply would catch up, he said here’s no telling when. “There’s nothing to compare it to is the real answer to that, because it has no predecessor product. It’s not replacing something that existed.”

The popularity of the iPhone X was on display with long lines Friday after September’s more muted availability of the iPhone 8 and 8 Plus.

Apple shares were sharply higher Friday, so much so that the company’s stock market value was at one point above $900 billion.