People change the spelling of names (both first and last) all the time and go form using a given name to using a nickname all the time.
Also people move all the time and may show say on 1900 roll and then not show up again until 1930 and that is more common the further back on the rolls you go.
Good Example is I have a relative on the 1860 in his father's household, and on the next roll of 1870 he is 16 and living with another family to help out on the farm.
Also people changed the last name legally and that does not show up in the census records.
The census records are a God send on research, but one needs to double check things all the time.
I have one person I have check into go form using Isaac to Bob, I have had a person that I know lived between 1884-1945 not show up on any census records except the 1900, I have seen spelling change like Allbright to Albright etc.
Any work in this field has to be done carefully, and it is much harder when you start using the 1840 rolls.