As a result of lumping dental plan enrollments into overall health insurance sign-up figures, White House officials overestimated the number of people who signed up for Obamacare by roughly 400,000.

Last year’s rollout of the Health Insurance Marketplaces was fraught with back-end bugs, delaying insurance enrollment for millions of Americans. The new errors reveal some kinks have yet to be worked out. Instead of the 7.3 million people the government estimated were enrolled in September, officials in Congress tallied the numbers and noticed they were closer to 6.97 million. Follow-up estimates showed a 7.1 million estimate was actually 6.7 million, according to Sylvia Mathews Burwell, Secretary of Health and Human Services.

The errors were uncovered by Republican investigators for the House Oversight and Government Reform Committee, using data from the U.S. Centers for Medicare and Medicaid Services. On Thursday, the investigators’ office released a report calling for an explanation behind the “padded numbers,” implying the figures were purposely misleading.

“Faced with large numbers of Americans running for an exit from Obamacare, instead of offering the public an accurate accounting, the Administration offered numbers that obscured and downplayed the number of dropouts,” Committee Chairman Darrell Issa said in the statement. “Now they’re saying this was just a ‘mistake.’”

Burwell, for her part, hasn’t explained how the error occurred. The HHS has only admitted to double counting dental plan sign-ups: once when people signed up standalone for dental coverage, and again if they signed up for health insurance and dental coverage. While much of the Marketplaces’ face has been repaired since the initial wrinkles, the White House concedes the back end is still under construction. September’s total figure was all the HHS could release, as no state or demographic figures were available at the time.

After a request from the Oversight and Government Reform Committee, Caitlin Carroll, a spokeswoman for the committee, said the errors stemmed from 289 Excel spreadsheets that, when tabulated, did equal 7.3 million people. But certain premiums were far too low to signal full health insurance plans — so-called “outlier” plans, which turned out to be dental plans. Burwell said future sign-up counts won’t include these plans in their overall estimates.

Total sign-up figures fell below the predictions made in 2013, when the Congressional Budget Office said seven million people would be enrolled by this year. Later estimates early this year lowered the projection to six million.

However, despite missing the estimated mark, the total enrollment figures might not even be the most accurate reflection of how much of the country is covered. For that figure, the best number to look at — the one “fundamental number,” according to Burwell — is the uninsured rate. According to Gallup, that number is down four percentage points this year, to a rate of 13.4 percent.