I think you've answered the questions in your post. It was a case of ignorance and inattention, sometimes referred to as (criminal) negligence.

It seems to suit some agendas, particularly in the UKicon, to pretend that the No.4 is unsafe, especially in 7.62mm. Use was made of this collective copulation and the report thereon for that purpose. There was a thread about this a few years back which you can probably find if you search for it.