Shares of Costco Wholesale (COST -3.87%) fell after the warehouse retailer posted comparable sales for May that were slightly below estimates.For a stock that's priced to perfection like Costco, that was enough to send the stock down 3.9% as of 2:01 p.m. ET. Costco's growth slowsCostco is one of the few retailers left that reports monthly comparable sales, as others have backed away from the practice, in part because they think it adds volatility to the stock. Investors sometimes overreact to the numbers. T ...