Jb, my backups (2 of them) are both 7 GB smaller than the original; my guess would be the difference is in things like swap files etc. that SD doesn't copy. A quote from Dave Nanian on the subject in the Shirt Pocket forums re things that SD doesn't copy:
QUOTE
VM Swap files, sleep images, temporary files, spotlight index -- all can be quite large.
I don't know why you're seeing a difference in "out of order items" - perhaps you should ask Dave on the SD forums?
http://www.shirt-pocket.com/forums/
Let us know what he says! 