Insurers that lost big bucks on the ACA market didn't necessarily do so just because of higher than expected claims, as is commonly assumed. The administrative costs that insurers incurred to develop and operate the new types of plans also cut into their profit margin.

The study found that most insurers only slightly underestimated the per-month cost of plan enrollees. Insurers projected an average per-member monthly cost of $406, and the actual cost ended up being $429.

However, after taking into account the federal reinsurance money that many insurers received to cover very high claims, insurers only ended up paying $9 a month more in claims than projected. Of course, that is hardly a meaningful assurance for the future, since the reinsurance program was only set up for the first three years of the ACA.

The study strikes an optimistic note in conclusion, positing that more insurers will figure out how to turn a profit on the ACA market as their understanding of the new insurance pool develops.

"All well-functioning markets have winners and losers, so it should be no surprise that some health insurers failed to succeed in the ACA's reformed market, especially during the first year," write the authors. "As insurers gain greater experience with these new conditions, it can be expected that their actuarial precision will improve and that large differences in financial performance will diminish."

Unfortunately, notes the study, part of the solution for insurers will undoubtedly be raising premiums in the next year.