Wal-Mart Stores Inc. (WMT) announced Wednesday it has lowered its estimated increase in September sales at stores open at least a year to 1.3 percent from 1.8 percent.

Wal-Mart, the world's largest retailer, said that the revision was "a result of the monthly reconciliation process that the company undergoes to determine its comparable sales percentage."

Its shares fell 91 cents, or 1.8 percent, to $48.55 in premarket trading.

Sales at stores open at least a year, known as same-store sales, are considered a key indicator of a retailer's health. Wal-Mart noted in its Saturday sales release that latest sales were being compared to sales last year influenced by buying after Hurricane Katrina.

The retailer said it would announce early Thursday its official sales figures for the five-week September period.