Updated from 8:53 a.m. EDT

Wal-Mart

(WMT) - Get Report

, less than a week after offering a disappointing monthly sales estimate, said its September results were even worse than expected.

The world's biggest retailer said Wednesday that its U.S. same-store sales for the five weeks ended Sept. 29 rose 1.3%. Last weekend, Wal-Mart said comp sales probably climbed 1.8%, a figure that was already toward the lower end of the company's earlier 1% to 3% forecast.

As a result of the monthly reconciliation process that the company undertakes to determine its comparable-sales percentage, Wal-Mart found that roughly 235 Wal-Mart stores and Sam's Clubs were incorrectly coded for the calculation. The revised comp sales estimate doesn't affect total sales.

Wal-Mart will issue its official sales release for September on Thursday.

Shares of Wal-Mart were falling 2.3% to $48.32 in morning trading.