Is this an actual birth cert? Without being able to see the birth certificate, it's hard to answer. If this was found on a copy of a register, it would suggest that the names were crossed through and the numbers assigned as transcriptions were made of the register, rather then any corrections were being made.
"The Mother of Robert George is shown as Amelia Black formerly Mason." This indicates that the mother's name at the time of Robert's birth was BLACK, but that she had been married previously at which time her name was MASON.
Take a look at the excellent BMD tutorial site at http://home.clara.net/dixons/Certificates/indexbd.htm
Hope that helps a little!