(Healthcare.gov/ABC News)

Four days ahead of the March 31 Obamacare enrollment deadline, the White House announced today that more than 6 million Americans have signed up for health insurance through the federal and state marketplaces. It's a milestone that President Obama touted on a conference call with health care navigators.

But 6 million is not the number the administration had originally envisioned.

The nonpartisan Congressional Budget Office predicted in May that 7 million people would enroll in Obamacare by the end of March. And at the time, it certainly seemed like the administration had adopted the CBO's estimate as its target number.

In June, HHS Secretary Kathleen Sebelius called 7 million a "realistic target." And an internal U.S. Department of Health and Human Services memo circulated in September heralded the 7 million enrollee goal.

Sebelius embraced the CBO estimate again in late September, telling NBC News, "success looks like at least 7 million people having signed up by the end of March 2014."

Two days later, healthcare.gov officially launched, beginning a rocky roll-out period marred by a series of technical glitches that complicated the enrollment process. The administration quickly began distancing itself from the 7 million goal.

In February, Vice President Joe Biden publicly backtracked:

"We may not get to 7 million," he acknowledged, "but if we get to 5 or 6 million, that's a hell of a start."

A few days later, Sebelius also sought to distance herself from the CBO number.

"First of all, 7 million was not the administration," the HHS Secretary told the Huffington Post. "That was a CBO, Congressional Budget Office, prediction when the bill was first signed. I'm not sure where they even got their numbers."

The CBO also revised its estimate, decreasing its forecast from 7 million to 6 million, a projection the Obamacare team (and its legions of volunteers) exceeded this morning.

Nevertheless, according to the White House today, President Obama held a conference call and thanked the health care volunteers for "all their hard work."

"The president encouraged the navigators and volunteers to redouble their efforts over the next four days," according to a White House readout of the call, "and leave no stone unturned in trying to bring affordable health coverage to as many Americans as possible by the March 31 deadline."

ABC News' Devin Dwyer and Mary Bruce contributed to this report.