Big difference between 1 tb drives

[SeaSwift]

I'm just curious as to why there is a big difference between my 1tb startup disk and my 1tb backup drive.
The backup drive was originally used for photos and videos until it got pretty full. I used Disk Utility to format the drive before using the previous version of SuperDuper! to clone my startup disk over to it. There was a difference between the two after that initial clone but, not as much there is now.
I have since updated SD! to the latest version on my Mac mini running OS X 10.8.5 and when I ran it last night using the SmartUpdate feature I seen that there was a difference of about 10 GB between the drives after it was done.

Any ideas on why the big difference between the two drives?

[dnanian]

As explained in the "What's going to happen?" section of the UI, a number of things aren't copied (that shouldn't be), including temporary files, swap files, sleep images, the Spotlight index, caches...this can easily add up to 10GB. (Just the sleep images, for example, can be as large as the amount of memory in the Mac.)