How does Oracle set the High Water Mark?

Oracle 9.2.0.4 Solaris 8

How does Oracle set the HWM? In the example below, I create a table and
insert one row. I would expect only one block to be used, instead Oracle
seems to have pushed the HWM up to 13 blocks. I can imagine that this might
be done to "pre-mark" rows as used during bulk inserts, but how does Oracle
decide where to set the HWM?

SQL> select * from dba_tablespaces where tablespace_name = 'EXAMPLE'; 
TABLESPACE_NAME BLOCK_SIZE INITIAL_EXTENT NEXT_EXTENT MIN_EXTENTS 
------------------------------ ---------- -------------- -----------
----------- 
MAX_EXTENTS PCT_INCREASE MIN_EXTLEN STATUS CONTENTS LOGGING FOR EXTENT_MAN 
----------- ------------ ---------- --------- --------- --------- ---
---------- 
ALLOCATIO PLU SEGMEN DEF_TAB_ 
--------- --- ------ -------- 
EXAMPLE 4096 65536 1 
2147483645 65536 ONLINE PERMANENT LOGGING NO LOCAL 
SYSTEM NO AUTO DISABLED 

SQL> create table t (col1 number(1)); 
Table created. 
SQL> analyze table t compute statistics; 
Table analyzed. 
SQL> insert into t values(1); 
1 row created. 
SQL> commit; 
Commit complete. 
SQL> select blocks, empty_blocks from user_tables where table_name = 'T'; 
BLOCKS EMPTY_BLOCKS 
---------- ------------ 
0 16 
SQL> analyze table t compute statistics; 
Table analyzed. 
SQL> select blocks, empty_blocks from user_tables where table_name = 'T'; 
BLOCKS EMPTY_BLOCKS 
---------- ------------ 
13 3 

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