Differential Backup Huge

I'm using Hard Disk Manager Suite 2012, but this happened with 2011 too, so I don't think it has anything to do with the version.

I'm using scripts to do the backups, and I do a full every 2 weeks and then differentials every day. The full backups are about 140GB in size, but the differentials are more than half that at 77GB in size. I'm baffled at how the differential can be so big. There's no way I'm changing that many files in a 24 hour period.

I have another machine I backup that has a full size of 127GB, but the differentials are normally about 1.5GB or less. That makes much more sense to me.

I'm trying to figure out if there's something running on the first machine that would be touching the archive bit on that many files.

Any ideas?