Options System Snapshot / Excluded Files page lists specific files which must be ignored by the System Snapshot tool when taking and comparing snapshots.
A certain list of Excluded Files is available by default. Make sure that there is a check mark next to those you want to be ignored when taking or comparing a snapshot; remove the mark next to those which must be processed.
You can use the buttons under the list to add new Excluded Files, modify existing ones or delete them. Please note that you don't have to delete a file just to stop considering it an Excluded one; instead, you can simply remove the check mark from the box next to it.
The button Default restores the default settings for this feature, including recovery of the items deleted by you and deletion of the ones added by you.