On some Census records it will say a person is 9 years old, but on the next census 10 years later that same person may now only be 18!
Two basic reasons:
(1) Human error.
(2) The census was not always taken on the say day of the year.
So, very easy for the age to vary by (at least) one year from census to census.
In addition to the answer already given, it largely depended on who provided the information for the census taker and exactly how the question was presented by the surveyor.