OK you are missing a common design feature in Backup Exec.
If you check out the table for how Backup Exec selects overwritable media in the Managing Media section of the admin guide - you will see that every possible configuration option in Backup Exec media management will result eventually in selecting recyclable(overwritable) media in any media set (not just the targetted one) - which is why the media can move between sets. This is a design choice as it was decided some time ago that most customers would prefer that if a backup is scheduled to start it is better that it finds suitable media to use in any way it can before the job is marked as a failure against no available media.
There is a way to stop this - you can partition slots in the library so that a group of slots becomes a unique device. You then target the specific jobs to the partition instead of the library and once this is done - the job cannot take media from a different device so will only use media in the slots assigned to that partition (with the media set protection still applying however). Note there is a downside to this in that unless each partition has a suitable number of slots then you are more likely to get a job failure because the job cannot find a spare tape because the only available ones are in a different parttion.
To Partition the slots in a library - go to the devices screen in teh Backup Exec Console and right click on the library itself.
Once the partitions are set up then edit each job to correct the target device so that it uses a partition instead of the library.
