The bad economy may be catching up with Apple.

While Mac sales have outpaced Windows PCs for months, January saw the reverse at U.S. retailers, including Apple Stores, NPD Group said Thursday. Mac sales fell 6% in terms of units sold compared with the same month a year ago, and 11% in terms of revenue.

Windows PC sales on the other hand were up 13% in terms of units, while revenue was flat as consumers sought out less-expensive models.

NPD analyst Stephen Baker said Apple sales fell for two reasons: price and the fact that the iMac desktop has not seen a refresh since April.

Baker believes that people who are shopping based on price will be more likely to choose a low-end Windows PC, whether a laptop or desktop, rather than pay more for a Mac. Apple fans, on the other hand, are probably delaying purchases until the economy improves.

"I don't think there's a lot of Apple people switching," Baker said. "We think people just aren't buying."

Indeed, Apple chief executive Steve Jobs has said that Apple buyers remain loyal to the brand and tend to delay purchases in hard times.

Apple has lowered the price of its MacBook laptop to $999, but that price still isn't low enough to compete with Windows PCs, said Baker, who believes Apple needs a $799 MacBook.

Apple, however, is unlikely to drop price much further. The computer maker, for example, has rejected the idea of selling a $500 mini-laptop to compete in the fast-growing netbook market where $300 systems are typical. "We don't know how to make a $500 computer that's not a piece of junk," Jobs told financial analysts in a conference call last October.

On the iMac, people interested in the desktop are probably waiting for Apple to update the line. Others are probably switching to MacBooks. The industry as a whole has seen people steadily turning away from desktops to laptops, which outsold the former for the first time last year.

"Some of the decline in iMac sales is cannibalization," Baker said. "People could be moving to a $999 MacBook."

NPD tracks sales at U.S. retail stores only, excluding online sales, so the final numbers on Apple sales could be different. As of the last quarter of last year, Apple reported a 9% increase year to year in Mac sales, outpacing the PC industry as a whole, which saw global shipments drop O.4%, according to IDC.