[Mimetic]

I think Spotlight is the problem. I found it refused to make the backup volume private. Then, I found I had to run Disk Utility and repair the volume...it found many errors.

I just could not get Spotlight to stop indexing the volume.

Finally, I just erased the entire backup, and I'm copying fresh. The backup showed less space than it should have; it's identical in size to the original yet showed no space free.