The decline comes in connection with the reporting of sales figures for the second quarter.
Sales at comparable stores did indeed rise by 9 percent during the period, in line with market expectations.
The company also says it expects sales in the third quarter to be around $1.6 billion, above analysts' expectations, but despite the positive forecast, the stock is falling sharply.
However, the decline should be seen in perspective, as the stock has been one of the year's big winners on the New York Stock Exchange, with a gain of 57 percent since the turn of the year.
