Oracle Database Discussions

Hello,
i am facing following problem with the automatic gather statistics job which is running every night.
The jobs starts when the weeknight/weekend_maintenance_windows opens, runs approx. 30 minutes and rebuilds statistics for around 270 objects. But when we check the status of objects next morning or immediate after jobrun, we still find hundrets of objects with stale statistics. We are not facing any heavy performance problems, but i want to know why we found stale statistics. The job uses 30 minutes of the 480 minutes window, so in my opinion there is plenty of time for all objects to create new statistics.

Is this normal behaviour?
Can't find anything in metalink.

Systeminfo:
Host: HP-UX 11.31
DB: 10.2.0.4

Regards
Ulli