number of page slots needed exceeds max_fsm_pages

I have a cron job that vacuums one of my dbs daily (Postgres 7.3.11)-
using 'vacuumdb -a -f -z -q -U postgres'. Currently I get an email
containing the folowing error messages:

NOTICE: number of page slots needed (38320) exceeds max_fsm_pages (20000)
HINT: Consider increasing the configuration parameter "max_fsm_pages"
to a value over 38320.
NOTICE: number of page slots needed (40912) exceeds max_fsm_pages (20000)
...
...

However if I run the command by hand it appears to complete without
error, any thoughts as to why this might be? The max_fsm_pages has been
set to 50000 in postgresql.conf.

Thanks,

David.