A relationship given on a document may not be quite as accurate or as precise as you would like. I’m listed as my great-aunt’s nephew on her death certificate–not her great-nephew. It’s a minor distinction, but still a distinction. A document may indicate two individuals are cousins, but that relationship may be first cousins, second cousins, or something other relationship. And sometimes a non-biological relative may be referred to by a term that is often used for biological relatives.
And keep in mind that some terms have changed their meanings over time.