“To people who care about particular diseases or conditions or provider groups, these don’t feel like the margins,” Mr. Weil said. “But at the end of the day, the core benefits are very standardized, and the differences are at the periphery.”

Some states have declined to choose an essential benefits package, saying that the law does not give them enough latitude. In those states, the default will be the largest plan available in their small-group insurance market, according to the Department of Health and Human Services.

Gov. Dave Heineman, Republican of Nebraska, chose an insurance plan with a high deductible as his state’s benchmark, reasoning that such lower-cost plans were popular in the state. But the Obama administration recently informed him that the plan did not meet the requirements of the law, he said.

“The point we were trying to make is that the minimum coverage should not be above what people need,” Mr. Heineman said. “The overriding concern is that the cost will be too great.”

Other states delayed choosing a benchmark plan on the grounds that the Obama administration had not provided enough guidance. Last month, the administration published a proposed rule that sought to answer outstanding questions.

The rule makes clear, for example, that insurers can substitute one covered service for another as long as they are in the same broad category and “substantially equal.” It clarifies that pediatric services, one of the 10 required categories, must be provided to everyone 18 and under.

States can still change or choose a benchmark plan, but they are running out of time. They generally have until Dec. 26, when the comment period for the proposed rule will end. So far, 23 states and the District of Columbia have chosen plans, according to Avalere Health, a consulting company.