Might be an idea to get the certificate to see who registered him. If it was his father, then he put his mothers maiden name down, more than likely that he got muddled and forgot that his wife wanted her mothers name!! I think the one in 1861 is giving the right age, as he appears registered in 1842 in December qtr, and on the census he is 19, because it was done before his 20th birthday.
My grandfather was supposed to have been baptised and registered using the mothers maiden name as well, but his father forgot, so it got left off!!!
There is another William who died in 1842, aged 8 so there may have been another one before your William, and they wanted to have the name kept in the family. It was quite a "tradition". if a baby/child died and the mother was expecting at the time, then the new baby if of the same sex, was often named after the one which had died. a bit macabre some might think, but that is the way it was.
Don't worry, looks as if you have all the right details, (not necessarily in the right order!!, to coin a phrase).