Hi Tieke,
My research suggests that your William (born 1858); was born before the marriage of his parents (Thomas Custance and Sarah Scrivener, 1860); hence, being named Scrivener at birth.
I'm guessing once he was old enough; he got his name legally changed to that of his father's; or 'just started using it'.
Your William was certainly the eldest brother of My Thomas (born 1860).
I have loads more info on the family; if you are interested, PM me with your Email address.
Jim