Another reason this could be happening is that some people don't start with census records and work back.
They may have heard that their ancestor was a famous person and happen to share the same surname.Or they come across an existing family genealogy on-line with the same surname and innocently think it's theirs.
If that's the case, this person probably didn't not know the names of their grandparents, or perhaps were even adopted.
Another scenario could be that the researcher's query does not supply enough specific information in which to receive help with their research.I'm sure you've seen some that read like this, "I'm looking for information on my grandfather whose name was John Smith. If you have any information on him please respond".
Most people are glad to help, and do, with WRONG information, based on what is posted.Then this person acts on it, and unknowingly and innocently, perpetrates (?) what might become a false statement of their ancestry.