"A big reason the CBO projections were so far off is that the agency overestimated how many people would lose insurance through their employers, which would force them into the exchanges.... In some markets, a shortfall in enrollment is testing insurers' ability to balance the medical claims they pay out with income from premiums.... Some of the first companies to enter the market made bad bets on how healthy customers would be, resulting in unprofitable health plans. Proponents say it's natural for new entrants to replace them, with better information and more competitive plans."