Why is there a difference in the character limit for VARCHAR2 in SQL and PL/SQL

The difference is that -- and /* */ can be used in a PL/SQL block, while REM[ARK] cannot. The following will work in SQL*Plus:

REM comment
-- comment
/* comment */
begin
   DBMS_OUTPUT.PUT_LINE('Test'); --comment
   DBMS_OUTPUT.PUT_LINE('Test'); /* comment */
end; 
/

These will not:

begin
   DBMS_OUTPUT.PUT_LINE('Test'); REM comment
end; 
/

begin
   REM comment
   DBMS_OUTPUT.PUT_LINE('Test');
end; 
/

The 11.2 documentation on all comment types has more comment information. The basics are...

You can enter comments in a script in three ways:

• using the SQL*Plus REMARK command for single line comments.

• using the SQL comment delimiters /*... */ for single or multi-line comments.

• using ANSI/ISO (American National Standards Institute/International Standards Organization) comments - - for single line comments.

The documentation also includes notes on four places that comments should not be used, but these do not include any further differences.

It would seem that Oracle at one time had plans to give a different definition to VARCHAR than to VARCHAR2. It has told customers this and recommends against using VARCHAR. Whatever their plans were, as of 11.2.0.2 VARCHAR is identical to VARCHAR2. Here is what the SQL Language Reference 11g Release 2 says:

Do not use the VARCHAR data type. Use the VARCHAR2 data type instead. Although the VARCHAR data type is currently synonymous with VARCHAR2, the VARCHAR data type is scheduled to be redefined as a separate data type used for variable-length character strings compared with different comparison semantics.

The PL/SQL User's Guide and Reference 10g Release 2 puts it this way:

Currently, VARCHAR is synonymous with VARCHAR2. However, in future releases of PL/SQL, to accommodate emerging SQL standards, VARCHAR might become a separate datatype with different comparison semantics. It is a good idea to use VARCHAR2 rather than VARCHAR.

The Database Concepts 10g Release 2 document says the same thing in stronger terms:

The VARCHAR datatype is synonymous with the VARCHAR2 datatype. To avoid possible changes in behavior, always use the VARCHAR2 datatype to store variable-length character strings.

The Oracle 9.2 and 8.1.7 documentation say essentially the same thing, so even though Oracle continually discourages the use of VARCHAR, so far they haven't done anything to change it's parity with VARCHAR2.