Probably one actual bad cell plus the one that is mated to it by the battery controller getting over-used by trying to make up for the bad cell. And then at the point where both of those cells are failing self-checks the controller shuts it down.
Dealership also is potentially restricted by their relationships and contracts with the sponsoring manufacturer(s) - if their service training and rules says 'never swap an individual cell' but does have a procedure for exchanging the entire battery assembly, or possibly the manufacturer only provides the entire assembly as parts and will not provide individual cells.. then 'swap the entire battery' is what they can offer if they want to continue being an Authorized Truly Authentic Certified FordHondaToyotaWhatever Service Provider, even if the mechanic working on it knows full well that they could probably just try to exchange out the actual failed cells.