My OH's father and sister both had the middle name MARSDEN and claimed not to have any idea why - they had asked, but been given a reply that it was none of their business. Or so his father said.
But it transpired that MARSDEN was the name under which their father (OH's grandfather) had been brought up, as his mother had married a Mr Marsden after his birth, and had another son, so the family name had become Marsden. He only changed it after enlisting in WWI, not quite sure why, perhaps it was at this point that his birth certificate emerged or his 'step'father told him he wasn't truly his next-of-kin. He then kept his birth name afterwards, and the whole family is called by it.
I discovered that the 'illegitimate' birth had taken place at the address where the future stepfather lived, and so there must be at least some question as to whether the stepfather was actually the biological father.
But what I've never been able to rationalise is that OH's father and aunt never realised that the mysterious MARSDEN name was actually their father's parents' married name.
Sadly I only discovered this after the death of OH's father so was never able to ask him about it - though OH's mother seemed very surprised at my findings.
Families, and the secrets they keep, eh?