The registrar has to ask the questions...
My sister who is a registrar preformed my marriage ceremony, and when it came to filling out the register, she explained that even though she knew all my details She still had to by law verbally ask me the questions and I had to verbally respond
When you consider any of the registrations, be it birth, marriage or death, the information given can be considered as here say...
It's assumed if a women is married, when she or the husband registers the birth of a child, that the father is the husband.... How many times has that been wrong?
Marriages are the same, how many people lied about their ages so they didn't have to gain parental permission to marry?
Deaths, are slightly different as a doctor will confirm the individual has actually died, just the rest of the information concerning full name and age can be slightly out due to what knowledge the informant has of the individual.
So how accurate our trees are, isn't just dependent on our skills as researchers, but if or how many fibs our ancestors told in there time