Many people approach family genealogy as if it were a precise science. Although it involves a lot of research, the "data" is often suspect, and definitive "proofs" are often lacking. Many of the government and church records that people rely upon to research their family trees contain transcription errors, not to mention deliberate misinformation.
Immigration officers often made up “English” spellings for the names of illiterate or non-English speaking immigrants; the same for census takers, priests, ministers and notaries. Also, people used to routinely lie about their true identities, birthdates and places of origin, especially former prisoners and indentured servants.
In my own family tree, there are four different spellings for our family name appearing in various government documents, along with differing multiple ages and birthdates recorded for the same persons. I know that some of my ancestors lied to census takers, and even on their marriage licenses.
It is also rather amusing that most family trees relate only to a paternal family name. Most of us have a rather naive perspective when it comes to the assumed marital fidelity of our ancestors. I recently read a medical journal article about paternity. It states that one in ten children born to married couples is not genetically related to the official “father”, and was actually sired by someone other than the man listed on the birth certificate. As a result, if one traces one’s ancestry back ten generations, then there is a good chance that there’s a bastard in there somewhere. Go back even further, and it’s quite likely that you’re not related to the men listed in your “official” family tree.
In addition, adoption was as common in the past as it is today, and often was not officially recorded. Many adopted children were never told that they were adopted.
So genealogy, like religion, is really a matter of faith.