Windows 7 - Backup & Restore - unmodified files still saved in next backup

Hello.

Since I use the Windows 7 Backup & Restore, I have a problem.

By default, the Windows backup is incremental, right ?

However, some files are backuped again in the next backup when they have undergone no manual modification compared to the previous backup.

They are essentially ISO files whose access date has been changed between two consecutive backups : a virtual PC installed on local desktop usually opens these ISO stored on a read-only remote desktop.

It's very annoying because some of these ISO weigh several GBytes and my backup HDD will be filled quickly at this rate.

The question I ask myself is :

Does Windows Backup based on last access file date to backup instead of last modification date ? Either the problematic ISO files are modified each time they are read remotly, despite that they are read-only on the remote computer that hosts them ?

Is there a convenient way to specify in the backup not to backup again these files until they have been manually changed ?

Thanks.

Timofort