Why does Nephi say he was a Jew?

Lehi and his sons, according to records of the plates of brass were actually descended from the tribe of Joseph through Manasseh (Alma 10:3), but politically they were Jews, since they had been under the rule of the kings of Judah before their departure (2 Nephi 30:4). The word “Jew” can be traced to the Hebrew Yehudi and was applied by the historian of the book of Kings (2 Kings 16:6; 25:25) and the prophet Jeremiah (Jeremiah 32:12; 34:9; 38:19; 40:15) to the people under the rule of the kings at Jerusalem.