GP Connect - Problem-Condition

[http://data.developer.nhs.uk/fhir/gpconnect-getrecord-phase3/Profile.GetRecordQueryResponse-Phase-3/gpconnect-problem-condition-1-0.html]

Why has a new coding system been introduced for category? Would the preferred valueset diagnosis|condition not provide the same information as the new valueset DIA/PRB codes? At very least, why not include the preferred code ‘diagnosis’ from valueset-condition-category, rather than creating a new code for an existing concept.