The problem is the exam structure. The first half is written questions, but typically accounts for somewhere between 50% and 66% of the exam. The remainder is negative marking MCQs. Therefore, you can either get 100% on the first half and 0% of the second half or get relatively high marks in both.

The MCQs are normalized according to some normal distribution. Therefore, the worst you can get is 0. However, if you score > 0, but you get the lowest mark in the class, you receive 0 marks. To make matters worse, much of the questions are written in such a way that, if you have in-depth knowledge of the subject area, the questions become ambigious, because the examiner might not have fully considered all the ways in which the true/false statement can be interpreted or reasoned (I think this would require some kind of perfect human being?). In effect: mess up on the MCQ's and you're toast.

Hope that makes sense!