I agree with Kay. In this case, we can consider ourselves lucky - there is a Wikitree convention that provides a reason.
Very few records, even to the present day, which contain a person's surname, were recorded by the individual themselves. Town clerks, clergyman, officers of the court, undertakers, newspaper reporters, etc., hear a sound, interpret it, and write down something. Who knows why they write down what they do.
As genealogists and human beings (perhaps with a trace of vanity and a need for validation?), we each have our own prejudices, as indicated by the variety of responses here. Sometimes it's "MY ancestor, MY spelling."
"Last name at birth," as mentioned (above and below) is just a choice made by Wikitree. An oft-used protocol is: 1) surname recorded on a birth record; 2) if PGM, use Anderson; 3) otherwise discuss with profile managers or on G2G.
"Good as most; better than some."