I have also seen a queue being checked for over sized hand luggage and yet - some were called out and told to pay and check it in - others were not. That's one queue for one flight from a small regional field that had plenty of time to check everyone and gain the approval of FR for doing so. But they didn't.
A cynic might suggest that the criterion used was to assess who looked like they were a soft touch and would put up/pay up without protest.