Interestingly, the gap has fairly steadily closed in percentage terms, as depicted below (click to enlarge).
While historically, it's not too uncommon for immigrants to make more than native workers (there are strong selection patterns in immigration (there's a reason some people come to the U.S.), and immigrant workers have far less access to welfare programs than natives and lower take-up of the programs they do have access to, meaning they must rely on labor income more).
