NLS_LENGTH_SEMANTICS, how does it work?

Question

The tilde character produced below is going to take up at least a couple of bytes and because my NLS_LENGTH_SEMANTICS is set to byte "x" is i assume only capable of storing 1 byte so shouldn't the code excerpt throw up an error?
When i run this code in TOAD (Tool for Oracle Application Developers) i don't get an error either BUT the character produced is "ã" as expected but in the SQLPLUS console i am getting "Ò", how can that be?
Cheers
Gus
DevSQL>set serveroutput on
DevSQL>select * from nls_session_parameters;
PARAMETER                      VALUE
NLS_LANGUAGE                   ENGLISH
NLS_TERRITORY                  UNITED KINGDOM
NLS_CURRENCY                   ú
NLS_ISO_CURRENCY               UNITED KINGDOM
NLS_NUMERIC_CHARACTERS         .,
NLS_CALENDAR                   GREGORIAN
NLS_DATE_FORMAT                DD-MON-RR
NLS_DATE_LANGUAGE              ENGLISH
NLS_SORT                       BINARY
NLS_TIME_FORMAT                HH24.MI.SSXFF
NLS_TIMESTAMP_FORMAT           DD-MON-RR HH24.MI.SSXFF
PARAMETER                      VALUE
NLS_TIME_TZ_FORMAT             HH24.MI.SSXFF TZR
NLS_TIMESTAMP_TZ_FORMAT        DD-MON-RR HH24.MI.SSXFF TZR
NLS_DUAL_CURRENCY              Ç
NLS_COMP                       BINARY
NLS_LENGTH_SEMANTICS           BYTE
NLS_NCHAR_CONV_EXCP            FALSE
17 rows selected.
DevSQL>declare
  2     x VARCHAR2(1) := compose(unistr('a\0303'));
  3  begin
  4     dbms_output.put_line(x);
  5  end;
  6  /
Ò
PL/SQL procedure successfully completed.

Answer 1

Sorry Justin the following output would have been more informative for my original posting...
DevSQL>select * from nls_database_parameters;
PARAMETER                      VALUE
NLS_LANGUAGE                   AMERICAN
NLS_TERRITORY                  AMERICA
NLS_CURRENCY                   $
NLS_ISO_CURRENCY               AMERICA
NLS_NUMERIC_CHARACTERS         .,
NLS_CHARACTERSET               WE8MSWIN1252
NLS_CALENDAR                   GREGORIAN
NLS_DATE_FORMAT                DD-MON-RR
NLS_DATE_LANGUAGE              AMERICAN
NLS_SORT                       BINARY
NLS_TIME_FORMAT                HH.MI.SSXFF AM
PARAMETER                      VALUE
NLS_TIMESTAMP_FORMAT           DD-MON-RR HH.MI.SSXFF AM
NLS_TIME_TZ_FORMAT             HH.MI.SSXFF AM TZR
NLS_TIMESTAMP_TZ_FORMAT        DD-MON-RR HH.MI.SSXFF AM TZR
NLS_DUAL_CURRENCY              $
NLS_COMP                       BINARY
NLS_LENGTH_SEMANTICS           BYTE
NLS_NCHAR_CONV_EXCP            FALSE
NLS_NCHAR_CHARACTERSET         AL16UTF16
NLS_RDBMS_VERSION              9.2.0.1.0
20 rows selected.
DevSQL>declare
  2     chars VARCHAR2(1) := length(compose(unistr('a\0303')));
  3     uni_chars number(1) := lengthc(compose(unistr('a\0303')));
  4     uni_chars_un number(1) := lengthc(unistr('a\0303'));
  5     bytes number(1) := lengthb(compose(unistr('a\0303')));
  6  begin
  7     dbms_output.put_line('Character length...' || chars);
  8     dbms_output.put_line('Unicode character length is...' || uni_chars);
  9     dbms_output.put_line('Uncomposed unicode character length is...' || uni_chars_un);
10     dbms_output.put_line('Byte length is...' || bytes);
11  end;
12  /
Character length...1
Unicode character length is...1
Uncomposed unicode character length is...1
Byte length is...2
PL/SQL procedure successfully completed.