I think the eighth one is the snapshot step itself, not an extra VM.
In the log you attached there are eight top-level tasks: one called snapshot VMs that takes all seven snapshots together, then seven backup VM tasks.
To me, that grouped step is what the Synchronize snapshots option adds (it's described in the XO 6.8 post), so I'm guessing the "All (8)" filter counts tasks rather than VMs.
Nothing extra got backed up, as far as I can tell. The number is confusing though, I agree. Does the backup report email also say 8?