In the cases mentioned in the threads it was related to the age that people needed to be before they were allowed to marry without parental consent. It seems that in a number of instances the person would add a few years to their age so they wouldn’t need parental consent to marry. The result of this is an incorrect age on the certificate making it difficult to go from there to the birth record. As the last poster in the thread says:
NEVER believe any age given on certificates or especially, the census
I spent today working through some New Zealand and UK birth records going through each year looking for names. It’s a tedious task! At least with the UK records you can often do a search on Ancestry or FreeBMD but the New Zealand records are not online.