Backup Exec 2010 R2 Media Sets

I have a question regarding Media Sets for Backup Exec 2010.  When I create a backup job and set it to use a particular Media Set "LTO2" the job set to use this Media Set will often use media associated with another media set even when media from LTO2 is loaded.  The media is not write protected and the retention period has expired.  An inventory shows the target media as overwritable and the job is configured to overwrite.  Any thoughts as to why this would occour?