A common occurance among Slavic immigrants and others is that some of their first names come from the pre-christian past and have no biblical counterpart nor a counterpart in any other language. So those wanting to appear more "American" pick names that arephonetically close to theirs or because others in their ethnic group made a certain name swap previously or just because they liked a certain name. For a long while immigrants to America could call themselves anything they liked. Once the federal government took over issuing citizenships maintaining a one's identity became a bit more important. I always try to list everyone by the name on their ship manifests. Ship manifests were always compiled in the ports of departure and most people had no reason to use any other name than the one on their personal documents.