Having read through this topic i guess any of us looking for an explanation of the occassional anomaly on birth certificates will start by checking whether the birth had been registered by dad 
I could never understand why my sister thought that our mum was two years older than her actual age because that was what mum would tell her. The strange part being that mum had never made a secret of her DOB. so all you needed was to do the maths - and sis was a mathetician 
The strange part was that growing up I'd always believed absolutely that my sister was five years older than me with the corresponding DOB and it was only when she died last year that I saw from her birth certificate that she was actually six years older all along
I've also always known the dob of all my close relatives, day, month, year, (all correct!) and so it's been surprising recently to find out that other people don't seem to notice large discrepancies and the conclusions one might draw from them...
Recently a researcher, T, contacted me to say that her sister in law, HC, was looking for a cousin, D, connected on her mother's side. D is a cousin of mine on my father's side.
This was interesting, because D's mother, H, was Irish, and died very young when he was a child, and nobody in the family knew much about her.
In the usual way, given that both T and myself are family history researchers, although neither of us is directly connected, we both set about finding out about the family.
In short, it turned out that D's mother, H, was born in 1925. However, her "father", was well known in the family to have "died in the war", by which was meant the first world war. The family were all highly respected Catholics, living in Ireland.
We found the death source - sure enough, it was in 1917. But nobody in the family seemed to notice the HUGE anomaly, which explained why H's birth reference had been so difficult to track down. Eventually I found her, under her mother's maiden surname.
T and I were quite worried about the implications of our discovery, particularly on the Irish Catholic side of the family. But HC is apparently "very vague about dates", even though one of the possibilities includes her own mother (aged 15 in 1925) being the parent of H.
It's amazing what you can just refuse to notice, if you set your mind to it.