Creampuff - Why did the pilot make the error?
Neatly and concisely put. The answer is as complex as human nature its self.
Can I leave aside my cultural bias and give you a straight answer, probably not.
As you stated, this is the remit of ATSB, and if I may be so bold, Mr Dolan did not carry out the investigation nor did Big John carry out the CASA Audit.
Up until about half an hour before top of descent into NFI, there MAY have been several systemic failings, after this point, it seems to me PIC had sole responsibility for the aircraft ending up in the water at near Norfolk and not on the hard at his alternative.
Those that subscribe fully to the "swiss cheese" paradigm must realize PIC was responsible for the last three or more holes lining up.
Em tasol, mi les pinis