Why postgresql 8.3.5 created temporary files of size less then work_mem on disk ?

Hello,

On a test postgresql server I have set work_mem to 200MB, and I have
stop/start postgressql.
I have also enable the  "log_temp_files = 0"

After restarting my server and executing some sql statements I see in
the postgres log:

LOG:  temporary file: path "base/pgsql_tmp/pgsql_tmp6655.10", size 122869324

So in what case temporary file of size less then work_mem are created on
disk ?


Yves D'Astous