The Obama administration's tally of 7.3 million paid enrollees in ObamaCare exchange plans as of mid-August increasingly looks like an artificial peak that hasn't been sustained.

The most telling data point since the Department of Health and Human Services announced exchange membership last month comes from California, where the 1.4 million sign-ups as of mid-April have dwindled to just over 1.1 million paying customers — a drop of 20.3%.

That decline is on the same scale as in Florida, where insurer rate filings in June showed 763,000 exchange plan members, down 22.5% from the 984,000 sign-ups reported by HHS.

California and Florida have seen their combined reported membership drop by 506,000.

If enrollment were still holding at 7.3 million, the combined 5.6 million sign-ups in every other state and D.C. through mid-April could have only dropped by about 200,000, or 3.8%.

Could California and Florida be such outliers? That doesn't appear to be the case based on the data available from a handful of states and one big insurer.

HHS Tally Full Of Grace

There is a possible explanation for why the HHS ObamaCare enrollment tally could have been accurate as of August, but somewhat overstated in September. Due to the law's 90-day grace period, those who last paid premiums for May could still have been deemed covered by the state exchanges as of mid-August.

So the HHS tally incorporated paying members who signed up after the end of open enrollment, but didn't exclude some members who stopped paying.

The next-biggest state with useful data is Washington, and its current tally has seen a bigger drop vs. open-enrollment sign-ups than Florida and California.

Washington is the only state that culled unpaid sign-ups from its reporting to HHS. Still, its current enrollment of 147,000 is off more than 10% from the official tally of 164,000 as of mid-April.

Yet, as of March 25, when the final enrollment surge was still going strong, Washington Healthplanfinder reported paid and unpaid sign-ups of 192,000. That means the current paid-up enrollment is down more than 23% from the sign-up total that is apples-to-apples with other states' reports from April.

According to an Arkansas Times report on ObamaCare, that state's paid enrollment (including those in a grace period) is 37,400, down about 14% from April's 43,446.

Minnesota's largest exchange player, PreferredOne, had 59% of total sign-ups through mid-April, or 28,600. But by July, enrollment fell 15.7% to 24,100. PreferredOne has chosen not to participate in Minnesota's exchange in 2015.

Aetna Sees Big Decline

The other significant piece of evidence that seems to suggest that enrollment is lower than HHS has said comes from Aetna (AET). The insurer, which is participating in 16 state exchanges along with its Coventry unit, has said its enrollment has fallen from 720,000 in May to "a little under 600,000," a decline of more than 17%.

While double-digit-percentage enrollment drops seem to be the norm, some smaller ObamaCare markets have seen less attrition.

The latest data from Connecticut highlighted by ACASignups.net shows current enrollment of 74,300, down 6.1% from 79,200 as of mid-April.

In Oregon, where technical glitches led to a longer open-enrollment extension than in most of the U.S., current enrollment of 77,900 is actually above mid-April's level of 68,300. Still, that is off 5.3% since hitting 82,300 in the first week of August.

If that same 5.3% attrition had been seen nationwide since HHS announced the 7.3 million figure in August, overall enrollment would be 6.9 million.

In updating its exchange enrollment, California revealed that just 81% of the 1.4 million who signed up in open enrollment eventually paid. Since then, enrollment has dipped slightly, from 1.14 million to 1.12 million.

Aetna management noted in a Tuesday conference call that they expect declines in its public exchange members through December. (Previously, the insurer said it expects exchange enrollment to end 2014 a little above 500,000.) That may be, in part, because the grace period lets people skip payments at year-end without any lapse in coverage.

Despite a lack of transparency from the Obama administration about how enrollment has evolved, there are many reasons why a person might drop coverage, some of them positive, such as getting a full-time job with coverage.

Washington state said that of about 24,000 no longer enrolled, one-third shifted to Medicaid.