Why does the postgresql function return an integer?

CREATE OR REPLACE FUNCTION get_zp(zp real default 70) RETURNS real AS $$
DECLARE
    percent_zp real := 0.15;
    zp_new real;
BEGIN
    IF zp > 70 THEN  zp_new = zp;
    ELSIF zp < 0 THEN  zp_new = zp;
    ELSE zp_new = zp * percent_zp+zp;
    END IF;
    RETURN zp_new;
END;
$$ LANGUAGE plpgsql;

execute

select * from get_zp(50);

why does the function return 50???