The Center for Budget Policies and Priorities had charted a Medicare spending forecast made in 2010 and another three years later. The difference between the two is $511 billion.

The dramatic drop in spending can mostly be attributed to the recent slowdown in health care costs: For three years now, health care costs have grown at the exact same pace as the rest of the economy instead of outpacing other industries.

"In recent years, health care spending has grown more slowly both nationally and for federal programs than historical rates would have indicated," CBO wrote in its most recent budget forecast. The agency noted that, in 2012, "federal spending for Medicare and Medicaid was about 5 percent below the amount that CBO projected in March 2010.

Because of that slowdown, "CBO made a series of downward technical adjustments to its projections of spending for Medicare and Medicaid."

The big question now is whether that arrested pace is the new normal or a temporary reflection of the recession. You can read more on that here, and also be sure to check out this great piece on the issue by Annie Lowrey of the New York Times.