In the blink of an eye, Obamacare enrollment numbers through August fell from 7.3 million to just under 7 million -- a level that dips overall enrollment under 2013 enrollment projections from the Congressional Budget Office. How'd it happen? In short, the administration combined Obamacare medical plan enrollment with dental plan enrollment for those August numbers -- while previous reports had kept the two numbers distinct.

Here's how the Department of Health and Human Services reported enrollments in a report last April. This shows only the last line of enrollments, but the number of plans with data on "metal level" (that is, the quality of the plan), is fairly close to the overall number of enrollments. Underneath, the number of people who got standalone dental coverage. 8 million; 1.1 million.

That 8 million figure was celebrated by the administration, largely because in the weeks leading up to the March 31 deadline for 2014 coverage, it didn't look like enrollments would pass 7 million, much less 8. And 7 million, as mentioned above, is what the CBO figured enrollment would look like this year. (At least originally. That 7 million figure is fairly complicated, and, in the wake of the fiasco last year, was adjusted downward to 6 million.)

In September, with the end of the federal government's fiscal year approaching, officials said that 7.3 million people were participating in exchanges. But as Bloomberg's Alex Wayne first noted, that includes 6.97 million medical plan enrollees and just over 300,000 dental. Our Jason Millman also outlined how a recent report of 7.1 million enrollees was similarly wrong.

Using old reports, we put together this chart showing both the increase in enrollment and the extent of attrition since the enrollment deadline.

The bars for August show the blended number that was originally presented and the split that was reported on Thursday. Notice that the green bar doesn't quite reach the 7 million line, for what it's worth.

It was always expected that enrollment numbers would drop after the enrollment period ended in March, and the extent of the attrition is close to what one independent analyst cited by Bloomberg expected to see. It seems, though, that the administration changed the way it talks about enrollment numbers in order to partially mask how steep the decline actually was.

Update: Sylvia Burwell, the head of HHS, tweeted something of an apology.