That's a good point, Koromo. I just think we have to assume that the enumerator was transcribing householder-completed census forms into his/her enumeration book unless there is any evidence to the contrary, which there won't usually be.
At a guess, literacy rates in 1841 were probably at about 40-50%, depending on location, so the many illiterate would have had to rely on third parties (if not, say, a neighbour then the enumerator) to complete their census form.