Steady improvements in American life expectancy have stalled, and more Americans are dying at younger ages. But for companies straining under the burden of their pension obligations, the distressing trend could have a grim upside: If people don't end up living as long as they were projected to just a few years ago, their employers ultimately won't have to pay them as much in pension and other lifelong retirement benefits.

In 2015, the American death rate—the age-adjusted share of Americans dying—rose slightly for the first time since 1999. And over the last two years, at least 12 large companies, from Verizon to General Motors, have said recent slips in mortality improvement have led them to reduce their estimates for how much they could owe retirees by upward of a combined $9.7 billion, according to a Bloomberg analysis of company filings. "Revised assumptions indicating a shortened longevity," for instance, led Lockheed Martin to adjust its estimated retirement obligations downward by a total of about $1.6 billion for 2015 and 2016, it said in its most recent annual report.

Mortality trends are only a small piece of the calculation companies make when estimating what they'll owe retirees, and indeed, other factors actually led Lockheed's pension obligations to rise last year. Variables such as asset returns, salary levels, and health care costs can cause big swings in what companies expect to pay retirees. The fact that people are dying slightly younger won't cure corporate America's pension woes—but the fact that companies are taking it into account shows just how serious the shift in America's mortality trends is.