Why would the first backup on a new time machine shared drive be almost 100GB larger than the data stored on the disk? I understand why subsequent backups will take more space, but this is a brand new backup and there's only been a few GB of disk changes since the backup started.
The picture below shows a 604GB backup of a 1TB system disk with 490GB available.
UPDATE: the backup didn't finish until it claimed to have backed up more than 700GB.
UPDATE: Although a big portion of my disk is Parallels VMs, Parallels wasn't running during the backup and hasn't been running for weeks. (I only need VMs to occasionally run some Windows software every month or two.)
What could be causing this? I'm running MacOS Catalina 10.15.4 if that's relevant.