Think about that marriage record for your great-grandparents that gives the names of their parents. Think about that 1900 census form that provides the place of birth for the parents.
Do you really know who provided that information? Did the bride give some of the groom's information? Did the groom provide some of the groom's information? Did the wife in a 1900 census enumeration simply guess at where her in-laws were born? Very possible.
And since most of us were not there when our great-grandparents' wedding or when the 1900 census was taken, the only thing we can do is conjecture about who answered those questions.
Is the informant the problem?