Short answer: There are a few common reasons: you used an out-of-network provider, the service was billed separately from the office visit, the visit turned diagnostic (for example, a screening that found a problem and led to more testing), or the service was not on the official recommended list. Free preventive care is guaranteed only in specific circumstances.
Plans must cover recommended preventive services without cost-sharing, but a $0 bill is not guaranteed in every case. Under federal rules, if a preventive item is billed separately (or is tracked as individual encounter data separately) from an office visit, the plan may charge cost-sharing for the visit itself. If the main purpose of the visit was not preventive, the visit can also be billed.
Other frequent reasons for a charge: the provider was out-of-network, or what began as a screening became diagnostic. A colonoscopy that removes a polyp, or a screening that leads to follow-up tests, can be coded differently and billed. If you think a charge is wrong, ask the provider how the visit was coded and appeal with your plan if needed.