Here is what’s happening:

King v. Burwell, the case to be heard Wednesday, centers on the legality of insurance subsidies being provided in states that use the federal HealthCare.gov platform. Some congressional sponsors of the health-care law have said that they clearly intended to make subsidies available to individuals in all states, regardless of whether states used their own or the federal insurance exchange.

In op-eds and amicus briefs, several members of Congress have argued that an Internal Revenue Service rule proposed in August 2011 and finalized in May 2012 that extended subsidies to individuals in both state- and federally run insurance exchanges was consistent with their intent at the time the health-care law was passed. The Congressional Budget Office “came to the same conclusion,” five lawmakers wrote in the Washington Post last October. The legislators say that because CBO assumed that subsidies would be available in all 50 states, as expressed by CBO scores for the bill when it passed, Congress’s intent was clear. But on a different issue of interpretation, several of the law’s authors undermined that logic.

The issue that prompted the about-face involves the “family glitch” related to eligibility for insurance subsidies. If one parent is offered health insurance through an employer, the entire family does not qualify for subsidies to purchase coverage through the marketplace. In March 2010, the same week the health-care bill was signed into law, the Joint Committee on Taxation issued an analysis of the legislation that said, in part, that even though “family coverage costs more than 9.5 percent of income, the family does not qualify for a tax credit regardless of whether the employee purchases self-only coverage or does not purchase self-only coverage through the employer.”

The same August 2011 proposed rule that prompted King v. Burwell also included Treasury proposals to codify the “family glitch,” consistent with the March 2010 technical explanation provided by the Joint Committee on Taxation. Yet Reps. Sander Levin and Henry Waxman—who, respectively, chaired the House Ways and Means Committee and the House Energy and Commerce Committee when the ACA was passed—wrote to Treasury in December 2011 complaining about this interpretation of the statute. Their letter argued that the Treasury interpretation of the glitch was “simply incongruent” with congressional intent and a “wrong interpretation of the law.”

When it came to the exchange subsidies, the Congressional Budget Office undertook no textual analysis of the statutory provisions at dispute in King v. Burwell. But the Joint Tax Committee did. It released a contemporaneous analysis of the provisions at issue with respect to the “family glitch.” Although Mr. Levin and Mr. Waxman say CBO’s silence suggests a presumption that subsidies should be available in all 50 states, they disregarded the contemporaneous analysis by the Joint Committee on Taxation.

Now, the former House committee chairmen could have been unaware of the JCT analysis at the time the law was passed. They could wish to argue for the most generous subsidy regime possible, regardless of the law’s technical specifics. There may be some other policy or political explanation.

But this situation highlights the pitfalls of claims regarding a law’s intent. All types of retrospective analyses could turn into self-justifying ones—which may provide little use to courts attempting to discern what a statute actually means.

This post was originally published at the Wall Street Journal Think Tank blog.