In summary, the terms Idiot and Imbecile were supposed to be used to describe, respectively, a person with a congenital disability that made them entirely dependent on others, and someone who descended into a such a state in later life (most likely through senility or what we would now describe as Alzheimer's).
But there were two main issues with that:
a) Even in 1901, the term idiot was pejorative and therefore census givers avoided using it. This is borne out by reports that when the term was replaced by feeble-minded in the 1911 Census, reports of mental disability increased dramatically.
b) The Census forms were completed by people who may themselves have been barely literate and may not have understood the distinction.
Within our own family tree we have a couple of examples of young adults described as imbeciles who probably had a congenital disability and should therefore have been described as idiots.