First of all you should know that there are different levels of distinctions. From what I can assume, you are talking about the World Distinction. Apart from that, there are 12 levels of distinctions.
It is NOT necessary that you'll achieve a distinction by scoring 100% marks. It can only be done if you are the ONLY ONE who scored 100% marks. When there are multiple candidates scoring the same highest marks around the world, then ONLY ONE is chosen from them whose paper is the best among them. By best I mean, the Most Neat Paper, Minimum numbers of errors done, Calculator model (In math related subjects), etc etc.
Just study hard and you'll get there.
Best of luck!