how to get accurate values in pg_statistic

Hi,

Last week I reported getting different execution plans for the same
query against the same database. I did further investigation. After
building the database, I did:
vacuumdb -z DBT3
psql DBT3 -c "analyze supplier"
psql DBT3 -c "analyze part"
psql DBT3 -c "analyze partsupp"
psql DBT3 -c "analyze customer"
psql DBT3 -c "analyze orders"
psql DBT3 -c "analyze lineitem"
psql DBT3 -c "analyze nation"
psql DBT3 -c "analyze region"

Then I check the data in pg_class and pg_statistic and get the execution
plans.

I did this twice (say run_8 and run_9). I found that while the pg_class
are the same for run_8 and run_9, the pg_statistic output are
different. I posted the query, the execution plan, pg_class and
pg_statistic output at:
http://www.osdl.org/archive/jenny/

The difference in pg_statistic results in a different execution plan for
query 18.sql. In fact, I updated the pg_statistic table for run_9 with
the values got from run_8, then it gives me the run_8 execution plan,
and the cost is very close(within 1). This makes me to believe that the
optimizer picks the plan based on the values in pg_statistic and
pg_class.

But why the pg_statistic value changes each time I build the database?
Did I do something wrong?

Thanks for your help,
Jenny