"Apple is definitely the most challenging OEM [to forecast numbers for]," David Daoud, of IDC, said in an interview today.

But the figures illustrate how difficult the U.S. market is for all computer makers, including the high-flying Apple. "Absolutely, they're working in the same declining PC environment as every other OEM," said Daoud.

In the U.S., IDC estimated that PC sales were down 12.4% overall, with global sales down 8.6%. Gartner posted similar numbers: -13.8% for the U.S., -8.3% for the planet.

Even though analysts had expected a decline of some kind for the PC industry, Daoud called the magnitude of the downturn, "a big shock."

"A proliferation of factors led to this, including economic, political -- it's an election year in the U.S. -- and people waiting for [Microsoft's] Windows 8," he explained.