When you drop tables in Oracle 11g, the database renames the table making it a hidden table as well as all of the constraints. It does not free space in any tablespace. Then if someone drops a table and realizes that they should not have dropped the table, they can recover it from the recycle bin. You can turn off this feature database wide if you want. Here is the documentation on what it is and how it works.

Code:
http://download.oracle.com/docs/cd/B28359_01/server.111/b28310/tables011.htm
So it could be that you need to do PURGE DBA_RECYCLEBIN; to free up space in your tablespace.