Short answer: The ACA sets a federal floor, but states regulate their own insurance markets, so group-size definitions, rating rules, tobacco surcharges, and continuation laws differ by state.
Health insurance is regulated at both the federal and state level. Federal law (ACA, ERISA, COBRA, HIPAA) sets nationwide minimums, but each state’s insurance department adds rules on top: whether a small group is 1 to 50 or 1 to 100 employees, whether age or community rating applies, whether tobacco surcharges are allowed, the number of rating areas, and how mini-COBRA continuation works. That’s why an identical employer can face different participation, contribution, and rating rules depending on the state where the policy is issued. (Self-funded ERISA plans are generally exempt from these state variations.)