For every census there is supposed to be an official census date. It does not always work out that way as census takers cannot be at every home on the precise date of the census. While respondents were told to answer questions as of the census date, there is no doubt that some got confused and answered questions as of the date the census taker was at their home.
Sometimes the extra month (or two or three) did not change the answers.
But sometimes it did.