A bit of history may help. In the late 80s early 90s the ADRP commissioned a safety consultancy to review their current safety arrangements and make recommendations. I think the Piper Alpha inquiry had recently found the safety case approach quite compelling and the consultancy recommended MOD take the concept on board. The concept appeared in Def Stan 00-56 around the same time but perhaps wasn't well understood by many. I can recall confusion in PTs and it's perhaps not surprising that each ended up with a different SC approach and format. In hindsight it might be that more time could have been spent on the management of the introduction of the change. I suspect that there are still ongoing issues but that might be another thread.
EAP