I have looked at thousands of US census records over the years. After looking at so many images, it is very obvious to me that the only thing you can glean with certainty from the census images from 1850 through 1930 is the place in which the person is residing. The occupation of adult members of the household generally is correct as well.
In the earlier censuses, the census taker often spelled both the given name and the surname phonetically. Thus, from decade to decade you can expect to find the same person, enumerated with a different spelling of one or both parts of his/her name.
Marital status and place of birth, I find are often incorrect followed by the year in which an immigrant entered the United States. But the one data point I found most often to be incorrect is the recorded age of adult females.
One relative for whom I viewed the census images for each of the census years 1850-1880 aged at a slower rate than the ten year interval between each census. This caused me to think of my mother who at some point in her life decided to decrement her age each year. I think I was about the age of 12 when my mother was the same age as I was.
So, it appears that women have lied about their age for years. However, when I viewed the census images of the 1900 US Census I saw errors across the board. The errors are not limited to adult females, but included adult men and children. The census of 1900 was the first census in which the month and year of birth were included. In the earlier censuses and the 1910 through 1930 censuses only the age was provided.
In each census, the enumerator was instructed to record the age of the individual at the last birthday. The instruction was the same for the 1900 census but I found many month/year birth dates to be off by a year. The month recorded in the record matched with the birth month that I found in other records but the year was off by one.
I have been baffled by this and wanted to find out why this occurred as often as it did. I did a Google search. The search results revealed nothing. I am not sure if anyone has asked that question until now.