SQL & PL/SQL

SQL> select * from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Release 11.2.0.1.0 - Production
PL/SQL Release 11.2.0.1.0 - Production
CORE    11.2.0.1.0  Production
TNS for 32-bit Windows: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production

SQL> create table fullJoinTest1(col1 char(14));

Table created.

SQL> create table fullJoinTest2(col1 char(14));

Table created.

SQL> insert all
  2  into fullJoinTest1 values('aaa')
  3  into fullJoinTest1 values('bbb')
  4  into fullJoinTest2 values('aaa')
  5  into fullJoinTest2 values('ccc')
  6  select 1 from dual;

4 rows created.

SQL> select nvl(a.col1,b.col1) as aaaa
  2    from fullJoinTest1 a full join fullJoinTest2 b
  3      on a.col1=b.col1
  4   where nvl(a.col1,b.col1)= 'ccc';

no rows selected

SQL> create table fullJoinTest3 as
  2  select nvl(a.col1,b.col1) as "WhatIsType?"
  3    from fullJoinTest1 a full join fullJoinTest2 b
  4      on a.col1=b.col1
  5   where nvl(a.col1,b.col1)= 'ccc';

Table created.

SQL> desc fullJoinTest3
 Name          Null?    Type
 ------------- -------- ------------
 WhatIsType?            VARCHAR2(14)

SQL> create table fullJoinTest4 as
  2  select case when a.col1 is not null then a.col1 else b.col1 end as "WhatIsType?"
  3    from fullJoinTest1 a full join fullJoinTest2 b
  4      on a.col1=b.col1;

Table created.

SQL> desc fullJoinTest4
 Name        Null?    Type
 ----------- -------- --------
 WhatIsType?          CHAR(14)

My question is why nvl function changes dataType from char to varchar2 ?
Even case expression does not change dataType.
I insist that this is a nvl functions bug is not it.