The post-New Year's Day drop-off was also larger this year than last. In the first quarter of 2012, Apple sold 900,000 fewer MacBooks than it did the quarter immediately prior; the first-quarter 2011 decline was only 156,000.

Cook has regularly admitted the iPad's cannibalization impact, but has never speculated about how many Macs have not been sold because of the tablet.

"There is cannibalization clearly of the Mac by the iPad," Cook said in the January 2012 earnings call. "But we continue to believe there is much more cannibalization of Windows PCs by the iPad and there's many more of them to cannibalize. And so we love that trend."

Analysts don't know how significant iPad cannibalization is, either, although -- like Cook -- they suspect it's happening. "If there is cannibalization by the iPad, you would definitely expect it to be in notebooks," Ezra Gottheil of Technology Business Research said in an interview earlier this week.

And the relatively puny growth rate last quarter of Apple's laptop sales could be caused by something completely different.