Bgwriter and pg_stat_bgwriter.buffers_clean aspects

Hello.

I am trying to tune PostgreSQL write parameters to make writing operation fast on a heavy-loaded database (a lot of inserts/updates).
After resetting the pg_stat_bgwriter statistics (I do it by deleting global/pgstat.stat file and restarting PostgreSQL) I monitor the following:

# select * from pg_stat_bgwriter;
 checkpoints_timed | checkpoints_req | buffers_checkpoint | buffers_clean | maxwritten_clean | buffers_backend | buffers_alloc
-------------------+-----------------+--------------------+---------------+------------------+-----------------+---------------
                 0 |               0 |                  0 |             0 |                0 |             164 |          6653

So, you see that just after resetting the statistics we have a large value in buffers_backend. Why?
Documentation: http://www.westnet.com/~gsmith/content/postgresql/chkp-bgw-83.htm
says that "buffers_backend