Hi - slightly complex answer to this one!
The scope *was* all objects in the domain. However, as we had the (fat finger) issue with the 'review process' executing twice and generating duplicate approval tickets, and not wanting to start firing off emails to reviewing managers that one of their accounts had been rejected because they had approved only one, we modified it.
In essense, we reviewed the execution log, identified any accounts that we had seen being denied, then added only those accounts into the scope, so that we could run the job against accounts we knew had been denied only.
A bit messy (our fault - this is a public forum after all!) but we had to be sure at the time.
To be honest we've not had any issues since, and there is no other evidence of anything going wrong, so I'm happy to put this one down to a freaky conflict due to the duplicate scheduled job run, and some of these manual tweaks we were applying to control the impacts afterwards.