PITR - Some data is not recovered.

I'm testing PITR using pgbench and postgresql ver.8.0bata. 
I think that result of recovery is wrong.  My test procedure 
is as follows:
--------------------
I edited postgresql.conf for PITR and started the postmaster.
And I executed "pgbench -t 20000". % pgbench -t 20000

I did backup procedure before end of pgbench. % psql -c "SELECT pg_start_backup('label1')" % cd $PGDATA % tar cf
/tmp/back.tar./* % psql -c "SELECT pg_stop_backup()"
 

restore and recovery % pg_ctl stop % cp -R $PGDATA/pg_xlog /tmp/. % rm -rf $PGDATA/* % cd $PGDATA % tar xf
/tmp/data.tar
 
 create "recovery.conf" file
 % rm -rf $PGDATA/pg_xlog % cp -R /tmp/pg_xlog $PGDATA/. % pg_ctl start 

check data after recovery % psql -c "SELECT count(*) from history"   count   -------   19999  (1 row)
------------------------
Number of records should be 20000, but result is 19999.
I found lack of data that was inserted near backup time, as a 
result of comparing original "history" table and  "history" table which
was recovered.
Is my backup procedure wrong?

Thanks, 
Satoshi OKADA