pg_autovacuum not sleeping

We're using pg_autovacuum with PostgreSQL 8.0.3 and it isn't sleeping
as long as the claims it will.  For example,

  ...
  [2005-09-20 15:40:23 CDT] INFO:         last_analyze_count: 21735550; last_vacuum_count: 21735009
  [2005-09-20 15:40:23 CDT] INFO:         analyze_threshold: 40676; vacuum_threshold: 81352
  [2005-09-20 15:40:23 CDT] DEBUG:   347 All DBs checked in: 1428748500 usec, will sleep for 4886 secs.
  [2005-09-20 15:50:14 CDT] DEBUG:   Performing: VACUUM ANALYZE "public"."model_lastexport"

So, pg_autovacuum says it's going to sleep for 4886 seconds, but fires
up again after just under 600 seconds.

Can anyone explain what I'm seeing?

Thanks.

--
Brandon