(Photo by Karen Bleier/AFP/Getty Images)

The Obama administration recently inflated Obamacare enrollment statistics by as many as 400,000 people by including stand-alone dental plans in their official count, according to a House committee investigation.

The administration in September said 7.3 million people at the time were enrolled in health plans through the Affordable Care Act insurance marketplaces. The House investigation, first reported by Bloomberg, found that this number also counted people enrolled in just dental coverage, a change from how previous enrollment figures have been counted that the Obama administration did not disclose. Without counting those dental plans, enrollment would have been 6.97 million.

That 7.3 million figure reported by the Department of Health and Human Services was down from the 8 million people who had signed up through the end of April. HHS hasn't provided a comprehensive accounting of why enrollment fell — such as how many people didn't pay their premiums or whether those enrollees found another source of coverage.

On Thursday, after news of the House investigation broke, the administration said that its total was "erroneously counted" in recent announcements.

HHS said it made this mistake twice. The agency overstated its September figures by about 400,000. Then in November, it reported an inaccurate figure again when it said 7.1 million people were enrolled at the time; the actual figure was 6.7 million.

"The mistake we made is unacceptable," HHS Secretary Sylvia Mathews Burwell wrote on Twitter on Thursday. "I will be communicating that clearly throughout the [department]."

Despite these corrected figures, HHS said it still aims to have 9.1 million covered in marketplace plans next year — which is about 4 million people fewer than the nonpartisan Congressional Budget Office had projected for 2015.

Rep. Darrell Issa (R-Calif.), chairman of the House Committee on Government Oversight and Reform, accused the administration of trying to obscure the number of people who had dropped out of the ACA insurance marketplaces, or exchanges, during the year.

"Only after repeated requests from Oversight Committee investigators did this egregious discrepancy become apparent," Issa said in a statement. "This Administration still appears to be calling its Obamacare transparency plan from the Jonathan Gruber playbook: dismissing the American public’s right to know with the same deceptive arrogance that helped them pass the bill in the first place.”

As my colleague Erik Wemple wrote just the other night, HHS has been especially stingy about providing enrollment data. Reporters have constantly pushed HHS for more up-to-date information, and the answer from HHS is usually the same: the agency can't provide real-time information because it needs to make sure all the data it's putting out is clean. Today's news obviously undercuts that argument. Meanwhile, some states running their own insurance marketplaces provide much more regular performance updates — like Massachusetts, which releases one every day.

Considering everything that's happened around Gruber's comments in recent days, there's no doubt this "error" looks especially bad for the administration.

This post has been updated with statements from Burwell and Issa.