In its new analysis, CBO did however compare the two scores for the 2012-2019 period during which they overlap. It found that passing the bill shaved $132 billion off the deficit, while repealing it would only add $119 billion – a $13 billion difference.

CBO explains that the variation is due to changes in the economic outlook and technical adjustments, as well as legislative changes since the law was passed.

“Some of the funding provided by the legislation enacted last March has been obligated or spent and thus would not be recovered by enacting H.R. 2,” CBO said. That includes millions of dollars in Medicare prescription drug rebates mailed to seniors.

“In addition,” CBO said, “some regulations implementing aspects of that legislation have been promulgated.”

In addition, legislation enacted since March has already changed the law. For example, Congress last year passed Medicare “doc fix” legislation that was paid for by recovering future overpayments to people who will get subsidies to buy insurance.

“That legislation was estimated to reduce net federal payments for subsidies through the [new] health insurance exchanges,” CBO said.

Separately, CBO responded to a request from House Budget Committee Chair Paul Ryan (R-Wis.) asking what repeal would do to the budget if the reform law doesn’t end up saving as much as forecast. That response is meaningful because Medicare’s chief actuary and the CBO itself have stated that budget analyses consider the law as it’s written, without taking into account possible changes as political pressure grows to avoid future cuts.

If a controversial tax on high-cost health insurance plans doesn’t go into effect as scheduled in 2018, and if only half of the law’s Medicare savings materialize, then repealing the law could shave up to .25 percent of GDP – about $600 billion – in the decade after 2021.