Mysterious name change

Magnus Broberg was my great great great grandfather. I recently found a photocopy of this document amongst family records. It shows that he was originally named Magnus Magnuson. As the document shows, he changed his named in 1872. This is particularly confusing because by this time, he had been in America for roughly 18 years, had served in the Civil War, and had fathered five children, including my great great grandfather. No one is left to explain why this happened and an explaination has been lost to time. John P. Hall was a longtime friend of his, who also served in the war with him. If I remember correctly, he was also a relative of Magnus's wife, and was later a pallbearer at Magnus's funeral.