I think something similar may be happening to these contacts I found. I see in their activity log that they successfully unsubscribe but during a data sync the unsubscribe field is changed from 'true' to 'false' and then the contacts were included in the next email send. I have included a screen shot
Who has access to edit the unsubscribe field? Could it be possible that your sales team is doing this? At a previous company it was an epidemic of sales people opting people back in because they "didn't really mean to opt out".
Everyone who has sfdc access can change this field and I know sales people are changing unsubscribed to 'true' when they come across people who say they have unsubscribed but salesforce is not reflecting that. I don't think anyone is opting people back in
From what it looks like there, Salesforce is the program that is changing the unsubscribe back from True to False. For some reason the original command from the sync didn't take in your instance of Salesforce. I would open a support ticket.
You could edit the Block Field Updates from the unsubscribe to block updates from Salesforce. But then sales people wouldn't be able to unsubscribe people manually, so that could cause a separate problem.