A new Gallup survey finds that the rate of uninsured adults in the United States is falling, but it's not clear what that really tells us about the implementation of President Obama's health care law.

At first glance, the news that the U.S. uninsured rate has declined to 15.9 percent would suggest that the law is making progress on one of its central goals. And it's certainly better news than other recent surveys suggesting that uninsured Americans were failing to embrace the law.

But the broader context makes it harder to draw any conclusions.

In the last three months of 2012, the rate of uninsured was 16.3 percent (only marginally different from the latest number), and it spiked over the course of 2013, reaching 18 percent by the third quarter, before starting to drop down again. (On a monthly basis, the rate had reached as high as 18.6 percent in 2013 and dropped to 16.1 percent a month ago).

So the story of the Gallup numbers over the past year is just as much a story of the uninsurance rate spiking in the summer/early fall of 2013 as it is a story of it declining at the start of 2014.

It's possible that people who were losing insurance as a result of changes in the law last year have been shifting to new plans on the Obamacare exchanges -- which would still be consistent with the theory that Obamacare hasn't made major gains among the long-term uninsured.

Additionally, taking the longer view, the 15.9 percent rate isn't historically that low. Measured monthly, the uninsured rate had been as low as 15.9 percent in early 2011. In 2008, the year Obama campaigned for president promising universal health insurance, the uninsured rate was consistently below its current rate, reaching a low of 14.4 percent quarterly and a monthly low of 13.9 percent.

Through Obamacare, American taxpayers have made a major financial commitment to expanding health insurance coverage. according to the Congressional Budget Office, the federal government will be spending a projected $40 billion in fiscal year 2014 and $2 trillion over the next decade to do so. The goal wasn't merely to bring the uninsurance rate to 2011 lows, or to 2008 levels, but to drive it substantially lower.

The deadline for individuals to purchase health insurance is on March 31, meaning that the number could decline further in the next Gallup survey. If the past is any indicator, official Census Bureau statistics on how this year's uninsured rate compares to last year's won't become available until the fall of 2015.