Source: Congressional Budget Office, Treasury Department

The federal government's 2013 fiscal year ended Sept. 30, though most of us were so busy focusing on the government shutdown that accompanied the new fiscal year that there wasn't much time to reflect on the year that had passed.

Now the Treasury and Office of Management and Budget is out with the final budget results. Surprise! The deficit fell quite a bit in 2013. The federal government took in $680 billion less revenue than it spent, or about 4.1 percent of gross domestic product. In 2012, those numbers were $1.087 trillion and 6.8 percent of GDP. That means the deficit fell a whopping 37 percent in one year.

This is the first sub-$1 trillion and sub-5 percent of GDP deficit since the 2008 fiscal year, which ended the very month that Lehman Brothers fell and a deep crisis set in.

What's behind it?

Most of all, there was more revenue. Government receipts totaled $2.774 trillion, up $325 billion from 2012, and rising to 16.7 percent of GDP from 15.2 percent. That reflects in part a stronger economy that increased income and payroll taxes. It also includes the expiration of a payroll tax holiday that increased tax receipts, and higher rates for upper-income Americans agreed to for this calendar year.

There was less spending, amid the drawdown of U.S. involvement in Afghanistan, lower unemployment insurance benefits due to an improving economy, and the enactment government enacted budget cuts called for in the 2011 debt ceiling deal, including the sequestration automatic spending cuts that began in March. Overall outlays were $3.454 trillion, the treasury said, falling $84 billion compared with the 2012 fiscal year. That fall moves government outlays from 22 percent of GDP to 20.8 percent.

It remains true that there are longer-term challenges facing the U.S. government finances, particularly around rising health-care costs. But the reality is that much of the conversation around debt and deficits is missing this basic fact: Deficits are, for now, falling fast. If anything, too fast. Just Wednesday, the Federal Reserve concluded a policy meeting with a statement that asserted, as it has in the past, that "fiscal policy is restraining growth" and that its forecasts are "taking into account the extent of federal fiscal retrenchment over the past year." Independent economists outside the government have reached similar conclusions, and now worry that deficits will fall so fast as to undermine the recovery.