The first quarter of 2016 was absolutely merciless to a lot of retailers. So when Wal-Mart ( WMT -2.58% ) reported earnings on May 19 that weren't downright awful, investors cheered and piled into its shares, sending the stock price up nearly 10%. Despite the bullish trading, there was one number that stuck out like a sore thumb in the earnings report: Wal-Mart's online sales grew just 7% year-over-year.