Reading is different. Comprehension is simply putting a bunch of answers together.
To get 15/15, my essay has to be essentially the best essay the examiner sees on the day. My essay will never get the perfect coherence and analysis to get full marks. It requires the intellectualism of a person who has displayed a passion reading and writing for the last several years. There's somewhat of a threshhold for my inherent writing style and despite the number of times I edit it, I will never get past the 14 point.
You might not get what I mean though.