John,
That is because the DARS (and DASC before it) regulates and makes policy. Airworthiness is managed by the Service Airworthiness Authorities who are officially the head of the Service Operations Branches (ACAS for the RAF, but delegated to DAAvn in the Army and I believe to Fleet ACOS AV in the RN - although the last one may be out of date).
If this has changed in the last year or two, please someone let me know!