Enrollment through the ObamaCare exchanges has been more sluggish than initially expected, with just about 12 million sign-ups likely this year. That’s better than the 10 million or so officials projected back in October, but far less than the 21 million the Congressional Budget Office estimated as the law was taking shape.

Part of the problem seems to be that people are finding ways to game the system by signing up outside of the limited annual enrollment period and then dropping insurance shortly after. The law has a number of exemptions that allow people to buy coverage at any time throughout the year, such as changing or losing a job, having a child, moving, and getting married.

The people who come in via those special enrollment exemptions, it turns out, are far more expensive to cover. In an earnings call last November, an executive with UnitedHealth, the nation’s largest insurer, said that people buying in outside of the standard enrollment period cost about 20% more.