Though the Department of Health of Human Services boasted this week that enrollment in Obamacare is consistent with the administration’s expectations, in reality, it has lagged behind original expectations for the second year in a row. The question is, can enrollment make a quantum leap in 2016 just to catch up?

When Obamacare was passed in March 2010, the Congressional Budget Office had projected that enrollment in the law’s insurance exchanges would grow from 8 million in 2014, to 13 million in 2015, and then jump to 21 million by 2016 (a projection that was reiterated in its most recent report).

But as of now, HHS says that just 10.2 million signed up and paid premiums (which only met HHS’s downwardly revised target). That means that the number of enrollees will have to double next year to meet CBO projections of 21 million.

Regardless of what the Supreme Court decides about the legality of federal exchange subsidies later this month, reaching 21 million will be a challenge. Especially because in the first two years, Obamacare may have already signed up the low-hanging fruit — those who had the greatest need for health insurance.