BOAC,
As I recall from my time in the Service, an "Error of Judgement" was classified as a genuine mistake and therefore was non culpable. On the other hand an "Error of Skill" was classified as culpable - i.e. the guy(s) had been equipped with the skills to get out of the particular situation, but porked it.
I don't know whether there were formal definitions of these for BoIs to consider, but BoIs made the call as appropriate.
Of course in this case, as no-one (with the exception of W & D!) actually knows what happened with absolutely no doubt whatsoever, the only sustainable verdict is CNPD.