Different last names for same person?

[mtlester]

I'm so confused. My great-Grandmother's baptism certificate shows a last name of "Bonaudi" on the left, says she is the daughter of Giavanni "Bonaudo", and on her marriage license in the U.S. after immigrating says her last name is "Bonandi," but let's start with the Christening... Why would her last name on the document be different from her father's? Can anyone help with this? Thank you!