The information provided in some records used by genealogists came right out of the informant's head. Grandma did not "prove" who were parents were when she married in 1912. Uncle Herman did not have to "show" that he immigrated in 1885 when answering 1910 census questions. Aunt Martha simply said her parents were born in Connecticut in 1880--they easily could have been born in Vermont for all she knew.
Remember that in many records the informant gave answers right off the top of their head, no "proof," no doublechecking, no verification.
And then 100 years later, their descendant encounters the information and assumes is all correct.