Glenn Kessler provides a useful clarification of CBO projections for enrollment in the exchanges under the Affordable Care Act, pointing out that the numbers refer to enrollment years. This means that a person who signs up for coverage beginning on April 1 will only count as three quarters of an enrolled person since they will only be covered for three quarters of a year.
However it is important to note that many more people will be signing up through 2014. While open season, in which anyone would enroll, ended on April 1, people who experience "life events" will be able to enroll at any time. "Life event" refers to anything that qualitatively changes your insurance or financial status. The most frequent life event is leaving a job, which happens to roughly 4 million people a month. Divorces, child birth, and deaths in the family are also life events.
This means that tens of millions of people will become eligible to enroll over the course of the year. Most will not be signing up with the exchanges (they will have other insurance options, such as a new job with insurance), but a substantial fraction will enroll through the exchanges. This will raise total enrollment above the level calculated based on the March enrollment numbers.