NEW YORK (CNNMoney.com) -- Wal-Mart Stores slashed its September sales results Wednesday, saying that about 235 of its Wal-Mart and Sam's Club stores were "incorrectly coded" to calculate its same-store sales last month.

Same-store sales are defined as sales at a retail store open at least a year. Wal-Mart calculates one full year of operation as 13 months.

A Wal-Mart spokesman explained that the error may have been caused when some stores that were open less than 13 months were included in its overall September sales results. Additionally, he said results of other older stores whose sales should have been tallied maybe were inadvertently left out.

Wal-Mart (up $0.09 to $49.55, Charts) shares were lower in afternoon trading on the New York Stock Exchange.

The world's largest retailer said sales at its stores open at least a year rose a much weaker 1.3 percent versus an earlier 1.8 percent increase. The company said total sales were not affected.

Wal-Mart had anticipated growth of 1 to 3 percent for the month. On Saturday, the retailer said its sales probably rose 1.8 percent during the month of September.

"Investors were already disappointed with the 1.8 percent result. Now this new, weaker result will only add insult to injury," said Mark Husson, analyst with HSBC.

Retail industry analyst Burt Flickinger of Strategic Resources Group said the mathematical problem may also have been linked to Wal-Mart accidentally including gas sales in the monthly sales figures.

Wal-Mart this year announced it would exclude gas sales numbers as part of its monthly comparable sales results.

Wal-Mart said it will issue its official sales for September on Thursday, when rivals Target (Charts), Costco (Charts) and Gap (Charts) are expected to release their sales numbers.

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