SQL & PL/SQL

SQL> select * from v$version;

BANNER
---------------------------------------------
Oracle9i Enterprise Edition Release 9.0.1.1.1
PL/SQL Release 9.0.1.1.1 - Production
CORE    9.0.1.1.1       Production
TNS for 32-bit Windows: Version 9.0.1.1.0 - P
NLSRTL Version 9.0.1.1.1 - Production

SQL> set null NULL
SQL> select a.aaa,b.bbb from
  2  (select 1 as aaa from dual) a
  3  left join (select 1 as bbb from dual
  4       union select 2 from dual
  5       union select 3 from dual) b
  6  on a.aaa=b.bbb and b.bbb=2;

AAA  BBB
---  ----
  1  NULL

SQL> select a.aaa,b.bbb from
  2  (select 1 as aaa from dual) a
  3  left join (select 1 as bbb from dual
  4       union select 2 from dual
  5       union select 3 from dual) b
  6  on a.aaa=b.bbb and 2=b.bbb;

AAA  BBB
---  ---
  1    1

Why different?