House Republicans poised to send a bill repealing most of the Patient Protection and Affordable Care Act to President Obama got some good news from the Congressional Budget Office, which, in contrast to a projection it released in June, now projects that undoing major parts of the landmark health law would reduce the federal deficit by more than half a trillion dollars over the next decade.

The nonpartisan agency had already projected budget savings of $474 billion last month, shortly after the Senate passed the bill, which achieves most of its budgetary effect by phasing out subsidies for the individual insurance exchanges as well as the federally-funded expansion of Medicaid.

Since that Dec. 3 estimate, the CBO has revised its estimate, projecting that the bill’s implementation would reduce the deficit by an additional $42 billion over the next decade, for a total of $516 billion.

And yet, it was only six months ago that the same agency estimated that repealing PPACA would increase the deficit by $353 billion over the next decade.

It similarly projected before the law’s passage in 2010 that it would decrease the deficit, and affirmed that estimate again shortly before the law was implemented in 2013.

Sigh. It’s hard to figure these number guys out.

Of course, the bill will be vetoed by President Obama, and neither House of Congress has the two-thirds majority necessary to override the veto. Thus, projecting the fiscal impact of the legislation is a largely academic exercise.

Showing that scrapping Obamacare would reduce the deficit was an absolute must for Republicans seeking to get a repeal bill on the president’s desk.

Not only does it go well with their political message that the health law is a fiscal disaster, but Senate rules dictate that only legislation that reduces the deficit can get approved through the “reconciliation process,” in which a bill cannot be blocked by a filibuster.

While the CBO estimates that the legislation itself would only reduce federal spending by $282 billion over ten years, it projects that additional economic variables that result from the legislation’s passage would reduce the deficit by an additional $193 billion.