We have been struggling for years to get this across to new genealogists and researchers, who insist that they found their information on a Census Index, and thus, it must be correct! I have been explaining away, and then re-explaining, and yet again, explaining, that an Index [of any kind] is open for human error. It is always best to look at the original document!
Leave it up to DearMyrtle to find the perfect way to explain it!
Okay, so folks, I'm not going to use my inept explanation on this one! I'm going to let you click here and take a look at Myrt's wonderful explanation! [By the way, if you're not reading DearMyrtle you don't know what you're missing! You can sign up at her blog and receive every new post she writes!]