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Differential Backups are acting as Full Backups

Level 4
It seems that my daily differential backup jobs are actually full copies each day.  My backup byte count is always between 500-550GB on tape on both differential and full backups.  I decided to check a known old file (xls) that hasn't been touched in years according to the modify date (March 2006).  This file among others according to the BackupExec are getting backed-up daily.  My last full backup was on Friday and the job was successful.  Why are these files continually getting backed up?

Backup Environment:
I have one backup selection list controlled by one policy that contains two templates (Differential Backup and Full Backup).

Selection List:
All Falmouth Servers

Tape Media Rotation (Manages both Daily and Weekly Tape Media Rotation)

Policy Template:
Differential Backup (Runs Mon - Thurs)
 >>Backup Method For Files: Differential - Backup changed files sicne last full - Using archive bit (does not reset archive bit)
Full Backup (Runs Fri.)
 >>Full - Backup files - Using archive bit (reset archive bit)

I even went as far to change the Backup Method For Files to Differential - Backup changed files since last full - Using modified time with the same results.

Any suggestions?

Level 6
Hi Todd -
It sounds like your setup is ok and as I read I was going to suggest what you already did and that was to attempt using modified time. Since you have already tested this, it sounds to me like some other application is modifying the archive bits or modified times on these files. This can sometimes be attributed to Third Party applications such as antivirus software. A next step could be to disable your antivirus during your Full and after your first differential to see if it is causing the bits or time to be set.
- Jared