Another possibility is that Warwick's manner might be acting as a filter for patients who are already determined to go the extra mile for their health, and causing any other patients to leave.

Perhaps any doctor would have better stats if they got to have the same pool of patients as those who remained with Warwick, without having other less motivated patients to drag them down.

We can't know if this is the case until we collect the data on patients who left Warwick's care.