They overlook eligibility. 

Which particular class of employees qualify for benefits and what does that class look like?

Imagine hiring an employee that starts Monday. And you choose to give them benefits on the first day of hire. Monday they are hired. Monday evening that employee goes to the dentist and uses $2000 of dental. And then on Tuesday, terminates. 

Those claims hit your plan. And your renewal will rise because of it. All those long-term employees you have, will be impacted by that employee who was there for a single day.

What should you do? Consider implementing waiting periods. Perhaps new employees should wait 3 or 6 months before qualifying for benefits. Most turnover occurs in the first few months of employment. Consider setting longer waiting periods for a class that turns over so frequently.

Insurance companies price their product on the assumption that they will provide an annual dental benefit and expect to receive 12 monthly payments for that annual benefit.