Your ancestor said it.
The census enumerator heard it.
The enumerator thought about it.
The enumerator wrote it down in his notes.
The enumerator put it in his actual census report.
It was microfilmed.
It was digitized.
And maybe somewhere in there it was transcribed or indexed, in which case it was read.
At any (or every) point in the process there could have been a misunderstanding. Multiple misunderstandings compound errors which easily resulted in entries that are a far cry from what was said by your ancestor.