But Instructors are mere mortals, whereas Examiners have Magical Powers that enable them to ascertain immediately if a Dual Flight logged six months ago was indeed flown with the holder of a valid EASA Instructor Rating.
Not really, it's just that we have demonstrated an understanding of our own privileges and Licencing regulations to a standardised level and the satisfaction of the authority. They trust us to show good judgement and integrity on their behalf.