Now here's a puzzle: I shall use aliases throughout ........
Fred & Ginger, the parents, are not yet married. Ginger is still married to Mr Bloggs but has left him to live with Fred. Divorce proceedings, citing Fred, have not yet been started by Mr Bloggs.
Child One is born to Fred & Ginger and is given three Christian names, the last of which is his mother's maiden name, and Fred's surname.
Child Two, born 18 months later and still before Ginger's divorce, is also given three Christian names. This time, the third one is his father's surname. Child Two's own surname is his mother's current (i.e. married) surname.
Whilst Child Two's name certainly explains why it took me so long to find him, I am curious as to why he didn't get his father's surname. The only possible reason that I can come up with is that Ginger registered his birth and could only prove her own surname, not Fred's.
Pointless speculation, I know, but........