Jaanna, the session parameters in Oracle SQL Developer are dependent on your client computer, while the NLS parameters on PL/SQL is from server.
For example the NLS_NUMERIC_CHARACTERS
on client computer can be ',.' while it's '.,' on server.
So when you run script from PL/SQL and Oracle SQL Developer the decimal separator can be completely different for the same script, unless you alter session with your expected NLS_NUMERIC_CHARACTERS
in the script.
One way to easily test your session parameter is to do:
select to_number(5/2) from dual;