There is not a strong correlation between results attained in Maths Competition and in VCE Maths. I'll take one competition I partook as an example - The University of Melbourne/BHP School Mathematics Competition. 7 worded problems, 3 hours, and you had to fully explain in writing how you conceived an answer. In the end I could only do 4 problems, and one of them took 3-4 pages of writing.
Careless mistakes are irrelevant in these circumstances, it's all about problem solving skill, speed, and the ability to communicate your conceptual understanding. Conversely all the questions on the VCE exams are easily solved, hence the ability to not make careless mistakes is paramount. In addition there is more than enough time to do every question, and you really only need to write down your answers and a bit of working out, which is nothing compared to the competition I referred to.
The two people who attained 50 in Spesh at my school this year never scored that highly on this competition, whereas Elvey-Price attained first place for it, and still got 43.
This is my theory - you don't need to be excellent at Maths to get a 50, you only need to be good enough. Once you have surpassed this level of "good enough," any additional mathematical prowess becomes irrelevant, and it becomes a matter of who can best avoid mistakes.