Retail sales just notched their best month since 2012 and the industry has added almost one million jobs since 2010. But the rosy headline stats obscure a more complex and potentially troubling story in retail—particularly for its employees.

The business of selling stuff is becoming much more efficient. Sales-per-employee have gone from $12,00 to $25,000 in the last two decades. That means that even as consumers spend more, we need fewer workers to stock shelves and process orders.

One reason retail has become so efficient is that more of it is happening across Internet cables rather than across registers. E-commerce is gobbling up one percentage point of total sales every two-and-a-half years. Call it the Amazon Effect.

And then there's the Walmart Effect. As I've reported, one Walmart worker replaces about 1.4 local retail workers, so that a county sees about 150 fewer jobs in the years after a Walmart opens its doors. Combined with the Amazon effect, this has dramatically reduced our need for retail workers to sell things, and so retail's share of the labor force, which peaked in the late 1980s, has been declining ever since.

This isn't the end of retail. But it is the end of some retail.