size inconsistencies?

[Dan Lester]

I was backing up a machine that had not been archived in a while (my bad), and was surprised to see SuperDuper churning through some 88.59GB as it backed up that hard disk. In fact "Get Info" on that disk (as well as the backup volume) gave that number as well. Um, that's strange, because that partition was only 80GB in size. You'd think that SuperDuper would have been reluctant to start. Now, when I look at that backup volume in Disk Utility when complete, it agrees that the partition size is 80GB, but says there is only 71.84GB "Used" in it. What's more, when I look at the breakdown of that volume in the Finder, it seems to total up to something like 20GB. So how does 20GB of stuff look like 88.59GB to System, and 71.84GB to Disk Utility?

I'm confused. This is probably more a System issue than a SD issue, but still ...