The uninsured rate went up in 2017, new Gallup data shows — the first time this has happened since the Affordable Care Act’s coverage expansion took effect.

This works out to an estimated 3.2 million fewer Americans having health insurance last year than did in 2016.

Dylan shared this chart earlier this week, but I want to talk about it in more detail because it raises a really interesting question: Why is the uninsured rate rising at the same time that the unemployment rate is shrinking?

Usually, these two data points move in tandem with each other. Because of our employer-based health insurance system, insurance rates typically rise when more people have jobs.

But the Gallup data suggests that something different happened in 2017. The unemployment rate declined steadily last year — from 4.8 percent in January 2017 to 4.1 percent last month — while the uninsured rate went up by 1.3 percentage points in the Gallup data.

What the heck is happening here?

The first explanation that popped into my head was policy: The Trump administration cut off key Obamacare subsidies in 2017, causing premiums on the insurance marketplaces to skyrocket. That could easily make health insurance unaffordable for people who buy Obamacare coverage on the individual marketplace.

But here’s the thing: The drop in uninsured rate wasn’t among that population. A more granular look at the Gallup data shows the rising uninsured rate is concentrated among Americans who earn less than $36,000 — a population that would qualify for federal tax credits to purchase coverage.

The uninsured rate for Americans who earn less than $36,000 went up nearly three times as quickly as the uninsured rate for those earning more than $90,000.

Trump’s decision to cut off Obamacare subsidies may be playing a small role in the rising uninsured rate, but it certainly doesn’t seem to be the entire story. The people who lost coverage at the highest rates qualified for either Medicaid (which charges no premiums) or generous federal subsidies that would have capped their monthly payment at a small percentage of income.

And other than that, the Trump administration didn’t actually make many health policy changes last year. It’s true that the tax bill did repeal the individual mandate, but that happened in the very last days of 2017 and can’t account for earlier Gallup data that showed the uninsured rate ticking up this past summer.

Right now the Gallup data is a bit of a mystery. There are questions about whether the Gallup sample is actually representative of the American population (Charles Gaba writes about that here), and we’re still waiting on government surveys of health insurance status to see if they replicate these findings.

Still, most experts think the Gallup data points in the right direction — the exact size of the decline in coverage might not be right, but the fact that there was a drop in the number of insured likely is.

”I suspect we’ll see an uptick in the uninsured but possibly not to the degree that Gallup [found],” says Jennifer Tolbert, director of state health reform at Kaiser Family Foundation.

The decline in uninsured rates could plausibly reflect the expectation or appearance of policy change, economist Craig Garthwaite argued when I called him to chat about the Gallup data.

He thinks the data might reflect the Trump administration’s constant talk of repealing the individual mandate — even if action didn’t come until the end of the year. That might have been the nudge Americans needed to decide it wasn’t worth renewing their coverage in 2017.

”This really seems to be about the choice to sign up rather than premium shock,” says Garthwaite, an associate professor at Northwestern University’s Kellogg School of Business. “You have the government telling you that the mandate is wrong, and maybe that alters your behavior.”

Tolbert sees other signs in the data that bolster this theory. She notes that some of the biggest declines in uninsured coverage are among young adults below the age of 25. These are the people you’d generally expect to be least attached to their coverage — the demographic that had low insurance rates before Obamacare.

”Especially among young adults, this is where talk of repealing the individual mandate may have influenced some people who would say, if I’m not going to get penalized I’ll take my chances,” she says.

This story appears in VoxCare, a newsletter from Vox on the latest twists and turns in America’s health care debate. Sign up to get VoxCare in your inbox along with more health care stats and news.