I had a similar gap with my grandad, he is the youngest in his family with an age gap of 9 years between between him and his older sister.
I had always presumed that he was just a late arrival in the family. Then when the 1911 census was released i found that there had been another sister that neither myself or my mother new anything about. When i looked into it i found out she had died about a year before my grandad was born.
When i did a bit more research i found out that some doctors would incourage a mother to have another child if she was still within child bearing age as a way to help with the grief.
My mother also said she was incouraged to get pregnant as soon as possible after suffering a miscarriage and this was many years later.
So my grandad could have been a happy late arrival or a sort of replacement for the child they had lost to illness. I will never know which it was but i am sure glad it happened as i wouldn't be here if it hadn't!