Only 6.7 million Americans were enrolled in Obamacare health insurance plans as of mid-October, not 7.1 million, the Obama administration admitted Thursday.

Health Secretary Sylvia Burwell acknowledged the error, calling it a mistake. The higher figure the White House reported earlier included people who enrolled in stand-alone dental plans. Previously the administration separated out enrollment in those dental plans.

"This mistake was unacceptable," Burwell said during a Facebook chat with MSNBC Thursday afternoon. "While we understand some will be skeptical, our clarity that this is mistake and the fact that we have quickly corrected the numbers should give people confidence. It is important to continue to focus on the fact that millions of Americans are getting affordable health care."

The overcount was uncovered by the House Oversight and Government Reform committee and first reported by Bloomberg News.

The Obama administration has long been very selective in the Obamacare data it releases. After open enrollment closed at the end of March, officials would only say that more than 8 million people had signed up for plans, but would not reveal how many had completed enrollment by paying their first month's premium. At that time, it said 1.1 million Americans had also signed up for stand-alone dental plans.

Officials finally released new figures in September, saying 7.3 million people had enrolled. In mid-November, they lowered the number to 7.1 million. The administration, however, did not say that these figures included the stand-alone plans.

Going forward, Burwell said, only those with medical insurance will be counted in Obamacare enrollment figures. The administration is sticking to its goal of getting 9.1 million Americans enrolled for 2015, about 4 million fewer than the Congressional Budget Office's projection.

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The correction means the administration missed the CBO's initial projection that 7 million would enroll. The CBO then lowered its estimate to 6 million after technical troubles stymied sign ups in the early months of open enrollment last year.

The administration has not said how many enrollees were previously uninsured, nor has it released how many people who've signed up for Medicaid were newly eligible under expansion.