The online index for NZ BDM has Thomas Earl ROBERTS as aged 83 years when he died (28 Oct) 1949.
That same online index has Henry Pim ROBERTS as aged 85 years when he died (27 May) 1952.
Surely, this is simply because, like so many BDM admin systems around the world, the family history details are informant driven, and BDMs don't have the funding or the need to employ auditing clerks to validate each detail on the BDM. The informant/s provided the age, as known to the informant. The informant gives the information to the best of their knowledge. So the family history information on a death registration is not first hand information. I would place much greater reliance on the accuracy of the information given, first hand, on a birth certificate, than the information on a death certificate.
Cheers, JM.