Enrollment in the Affordable Care Act's insurance exchanges grew again in January, but it grew at a much slower pace than the month before. How much of a concern that is for the law is open to interpretation.

In raw numbers, total enrollment in January was indeed a win for the Barack Obama administration -- 1.15 million people signed up for exchange plans, slightly more than the government projected for the month in estimates prepared last year. The new total, 3.3 million, is almost halfway to the Congressional Budget Office's initial estimates for 2014 exchange enrollment.

On the other hand, there are just two months left in the six-month open enrollment period, so getting close to those targets depends on people signing up at a constant or increasing rate. On the face of it, that's a reasonable hope: HealthCare.gov keeps getting better, advertising campaigns are ramping up, and Republican scare stories about people's personal information being vulnerable to theft seem to have fizzled.

That's why the 36 percent drop in new enrollees last month, compared with the month before, stands out. One possible explanation is that December's numbers were unusually high, as people rushed to replace coverage that was about to end. A less sanguine interpretation is that the people who most want to sign up for Obamacare have already done so, and persuading those who remain will become increasingly challenging over time.

We'll know soon enough which of those readings turns out to be right. In the meantime, the only game that matters right now is getting more people -- mostly young people, and young men in particular -- to sign up.

(Christopher Flavelle is a member of Bloomberg View's editorial board. Follow him on Twitter.)

To contact the writer of this article: Christopher Flavelle at cflavelle@bloomberg.net.

To contact the editor responsible for this article: Brooke Sample at bsample1@bloomberg.net.