On Tuesday, Walmart’s value, as reflected in its stock price, dropped by more than 10 percent. That’s nearly $31 billion. It had a bad quarter and in no small part suffered as a result of complications with its online inventory restocking system—it ran out of some items in demand and so couldn’t sell what it didn’t have. But there’s another part to Walmart’s apparent decline. Based on its failure to deliver stronger online sales, speculators rightly question if, in the long run, it can match Amazon.