Oct. 4 (Bloomberg) -- Wal-Mart Stores Inc., the world's largest retailer, unexpectedly lowered its estimate for September U.S. comparable-store sales growth to 1.3 percent after discovering mistakes in its calculations.

The company as recently as last week said sales for the month increased 1.8 percent. The Bentonville, Arkansas-based retailer said in a statement today that the error stemmed from 235 Wal-Mart and Sam's Club stores being ``incorrectly coded.''

Compared with rivals including Target Corp., Wal-Mart's sales are slowing as the important holiday-selling season approaches. Target, the second-largest U.S. discount chain, last week boosted its forecast for September growth to about 5 percent from a range of 3 percent to 5 percent.

``Their September was disappointing in what should be a very positive environment for retailers,'' said Rick Rubin, an analyst at Mercantile Bankshares Corp. in Baltimore, with $22 billion in assets including Wal-Mart shares.

Wal-Mart's sales growth has been blunted by the remodeling of about 1,200 stores that ``disrupted traffic flow,'' Donald Gher, chief investment officer at Coldstream Capital Management, said in an e-mail. His Bellevue, Washington-based firm manages $1 billion in assets, including Wal-Mart shares. Last September, Wal-Mart benefited from customers stocking up before and after hurricanes, Gher said.

In September 2005, Wal-Mart's same-stores sales rose 3.8 percent. Total sales for that month were $28.2 billion. The company had 3,931 U.S. stores at the end of August.

Shares of Wal-Mart rose 9 cents to $48.85 at 4:01 p.m. in New York Stock Exchange composite trading after falling as much as 2.7 percent earlier today. The stock is up 17 percent from its July 18 intraday low of $42.31.

Industry Gains

U.S. retailers' sales probably expanded almost 4 percent in September on falling gasoline prices and a jump in back-to- school sales, the International Council of Shopping Centers and UBS Securities LLC said yesterday.

``Wal-Mart is the one that is struggling,'' said Burt Flickinger, managing director of New York consulting firm Strategic Resource Group. Other U.S. retailers ``will do better,'' he said.

Gasoline prices have fallen 23 percent since July to an average of $2.31 a gallon in the week ended Oct. 2, according to the Energy Department.

Kohl's Corp., a discount department-store company, reported sales at stores open at least a year soared 16 percent. Most U.S. retailers, including Wal-Mart, are scheduled to report final September sales tomorrow.

Wal-Mart's $312.4 billion in sales last year were more than the combined revenue of the next five U.S. retailers.

Disruptions

Given the size of today's reduction, ``we felt an obligation to inform people as quickly as we could,'' Wal-Mart spokesman John Simley said.

Wal-Mart provides a monthly sales estimate for stores open at least a year on the last Saturday of each month, then releases a final figure the following Thursday. The company has never revised its number more than 0.1 percentage point in the two years it's given the Saturday estimates, Simley said.

The retailer is trying to get more of the 84 percent of Americans it says shop in its stores each month to buy more clothing, electronics and home goods. In the past year it has introduced exclusive apparel such as the George ME Line for men by designer Mark Eisen. It's also upgrading more than half of its U.S. Wal-Mart stores to trim clutter and improve restrooms and changing rooms.

To contact the reporter on this story: Lauren Coleman-Lochner in New York at llochner@bloomberg.net.

Last Updated: October 4, 2006 16:21 EDT