Rowid weird behaviour

Hi All,

I encountered a weird behavior on the rowid below

>select count(distinct dt.rowid) from table_a dt, table_b h
 where dt.sn = h.sn and dt.so = h.so ;

COUNT(DT.ROWID)
---------------
     10671

>select count(rowid) from table_a a where a.rowid in (select dt.rowid from table_a dt, table_b h
 where dt.sn = h.sn and dt.so = h.so );

 COUNT(rowid)
----------
   7237    ==> Less rows than the first query

I have 10671 rows of different rowid in 1st query, however, when I use the rowid to select the same table as in 2nd query, less count returned.