deleting the rows will not reset the highwater mark.
u have to truncate the table
or
move it ( within the same tablespace)

mag@itloaner1_local > select table_Name,tablespace_name from user_tables where table_name='EMP';

TABLE_NAME                     TABLESPACE_NAME
------------------------------ ------------------------------
EMP                            SYSTEM

mag@itloaner1_local > alter table emp move tablespace system;

Table altered.

----------------------------------------------------------------------

mag@itloaner1_local > create table backup_emp as select * from emp;

Table created.

mag@itloaner1_local > truncate table emp;

Table truncated.

mag@itloaner1_local > ed
Wrote file afiedt.buf

  1* insert into emp (select * from backup_emp)
mag@itloaner1_local > /

14 rows created.

mag@itloaner1_local > commit;

Commit complete.

mag@itloaner1_local > 

exporting and importing again with compress=y will help.
if you can afford some space in the same or any other tablespace, the easier method would
1. create table_new as select * from table_old;
2. truncate table_old;
3. insert into table_old ( select * from table_new);
4. drop table table_new;