In its annual ranking of the largest retailers by U.S. sales, the National Retail Federation has pegged Apple in 21st place in 2010 with over $18 billion in sales between its retail and iTunes stores (via The Next Web).
Apple's retail sales grew 32.3% year-over-year in 2010, second only to Amazon's 46.2% growth among the top 100 U.S. retailers. Interestingly, Apple's $18 billion in U.S. sales places the company one spot ahead of department store chain J.C. Penney, which will soon be led by current Apple retail store chief Ron Johnson.
The National Retail Federation's 2009 rankings published last year had Apple in 53rd place with only a little over $6.5 billion in sales. Those numbers would put Apple's annual growth at 175% instead of 32.3%, and thus it is unclear if something has changed in the methodology of the rankings or if there is an error in the data for Apple.
Update: We have heard back from the National Retail Federation, which reports that the methodology for Apple has indeed changed between 2009 and 2010. The new numbers include iTunes Store sales and direct-to-consumer sales through apple.com, whereas the previous numbers had excluded those sales. Consequently, the 32.3% annual growth is calculated as if iTunes and apple.com sales had been included in the 2009 data.