This may be an issue with the based Immunization resource from which the US Core is derived. However, from a modeling standpoint, the reportOrigin should indicate from where the information originated. Thus Immunization.primarySource (boolean) and Immunization.reportOrigin should be a single element and the choices include primarySource, SchoolRecord, parents, etc.  Creation of separate concepts seems awkward and leads to separate fields in the clinical software to indicate a single concept.  The primarySource is the one administering the vaccine as stated, but there is no clear reason for separating the primarySource as a different element - it should just be a choice in reportOrigin to simplify collection and data sharing.