Following up from previous question.
I've run the course of troubleshooting that I know how to do.
I've a multisite, http://www.fmsreliability.com that I'd like to create four backups on my dropbox account. It seems that after four snapshots, which is the limit I've set, it does delete the oldest backup on the server, yet doesn't delete the oldest dropbox backup.
I deleted all backups on the server and dropbox, deleted the snapshot and created a new one. Running every 5 minutes it seemed to work as described above. Now have it set backup twice weekly and will see if the behavior continues.
At the moment, maybe it's a feature request - delete the oldest on dropbox or google drive would be great.