why 'DBSNMP'and 'SYSMAN' are found when I tried to get the valeu of sys_context('userenv','session_user')?

I want to find the value of the 'sys_context('userenv','session_user')'when the user longon of database,and I wrote the folloing statements:  

create user t identified by t;

grant connect,resource to t;  

create user t1 identified by t1;

grant connect,resource to t1;    

conn t/t;

create table u (username varchar2(20));

grant select,delete on u to t1;  

conn system/manager as sysdba;  

create or replace trigger user_logon

after logon

on database

begin

insert into t.u values(sys_context('userenv','session_user'));

end;

/  

and then:  

SQL>conn t1/t1;

SQL> select * from t.u;  

USERNAME

T1  

SQL>conn t/t;

SQL> select * from t.u;    

USERNAME

T1

T

SQL> grant insert on u to t1;

SQL> select * from t.u;  

USERNAME

T1

T

SYSMAN   now 'SYSMAN' is found, why? and sometimes, it is 'DBSNMP', is there some kind of regulation about the emergence of 'SYSMAN'and 'DBSNMP'£¿anybody can tell me? Thank in advance.

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