Does user differ between v$session and sys_context('userenv','session_user')?

Hello PL/SQL Pros:

For the same SID, is there a difference between sys.v$session.username and sys_context('userenv','SESSION_USER')?  I'm writing a simple trigger and found this sample:

create or replace trigger on_logon after logon on database
declare
user_name varchar2(30);
os_user varchar2(30);
v_sid number;
v_su varchar2(15);
v_program varchar2(30);
v_ip varchar2(15);
begin
execute immediate
'select distinct sid from sys.v_$mystat' into v_sid;
execute immediate
'select osuser, username, program from sys.v_$session where sid = :b1'
into os_user, user_name, v_program using v_sid;
select sys_context('userenv','SESSION_USER') into v_su from dual;