WASHINGTON — The number of people enrolled in the nation’s marketplace health-insurance program was revised downward to 6.7 million people on Thursday after House investigators found that the original estimates for medical coverage included 400,000 people who had only dental coverage.

The gaffe sours the otherwise flawless Nov. 15 start of the 2015 enrollment period.

“A mistake was made in calculating the number of individuals with … marketplace enrollments,” said a statement by Aaron Albright, a spokesman for the Centers for Medicare and Medicaid Services. “Moving forward, only individuals with medical coverage will be included in our … enrollment numbers.”

It’s unclear why and how the Department of Health and Human Services mixed the two enrollment figures. The error, first reported by Bloomberg News, was uncovered by investigators from the House Oversight and Government Reform Committee.

The committee’s chairman, Rep. Darrell Issa, R-Calif., accused the Obama administration of engaging in a cover-up.

“Faced with large numbers of Americans running for an exit from Obamacare, instead of offering the public an accurate accounting, the administration engaged in an effort to obscure and downplay the number of dropouts,” Issa said in a statement. “Only after repeated requests from oversight committee investigators did this egregious discrepancy become apparent.”

The revised 6.7 million enrollment figure continues a downward spiral in signups for HealthCare.gov, the federal website for the Affordable Care Act, and the state marketplaces. It also brings the total marketplace enrollment below the original Congressional Budget Office projection of 7 million for 2014.

Earlier this month, Burwell lowered the administration’s projected enrollment for 2015 to 9.1 million. That’s down from the 13 million that the CBO projected.

After 8.1 million people initially signed up for 2014 coverage during the first marketplace enrollment period that ended in May, Medicare Administrator Marilyn Tavenner told the oversight committee that the number had fallen to 7.3 million.

She said the drop resulted, in part, from nonpayment of premiums, more people getting job-based coverage and people becoming eligible for coverage through Medicaid.