<p>The grade difference would be the reason. Usually people get a lower grade on the final than they got on the rest of the work all semester. To a professor it seems unlikely that you would have suddenly learned all the material for the cumulative final that you failed to learn in the smaller tests along the way. High 60’s to mid 80’s on the final is a big jump. Not an uncommon jump from one test to another, but an unusual jump for a cumulative final.</p>