Backup Exec 2010r3 with Partitions Configured

Hi,

I've inherited a backup solution that has been previously configured as follows

We have a Full backup configured to use tapes on slots [0001..0004]

A Daily Differential job configured to use tapes [0005..0008]

And a full monthly job configured to use slots [0009..0010]

The problem i have,  is that even though the jobs are configured to use the specific slots, 

i'm finding that when the jobs over-run,  instead of cancelling the job,  it's grabbing whatever media is available,  even if it is in another partition.

The way i understood the partions to work,  is that they will only ever use the tapes in that partition,  and will fail if there is no media available.

Can anyone help me understand why this is happening?

Thanks

Jon