Hi Ian,
The 1911 census that we are currently viewing which only shows one household per schedule are the individual schedules themselves, filled in by the householder. You can now also view the enumerator's summary book on the same site, at no extra cost, provided that you have already paid to view the individual schecule.
With regards to earlier census, the scanned pages are from the enumerators' summary books, completed by the enumerator from the individual household schedules. Every household received a schedule and the enumerator collected them on the Monday following the census. If it had not been completed, then the enumerator was instructed to complete it, having asked for the information from the householder. The household schedules for these census have been destroyed, so only the enumerators' summary books survive.
It does mean that there is already the possibility for mistranscription. If the enumerator could not read the householder's writing, he might get it wrong. Add the possibility that his writing was poor, faint, scrappy towards the end of a long day, it's no wonder that errors creep in. He might also mis-hear what is said to him, or put his own interpretation on it if the householder spoke with a strong accent.
Does that answer your question?
Nell