No, I think "luck" is the right word 
I disagree totally here. With respect to the original question, I agree with you Pi, the UMAT and ATAR test two totally different things and their scores cannot be correlated with any reliable accuracy. That being said, I strongly disagree that luck plays anything more than a negligible role in the UMAT - mind you, hard work and effort do not guarantee success either.
The biggest key in the UMAT, in my personal opinion, is the attitude and approach that the students takes with them into the exam. Personally if anything, considering how much the top-end students cram for their measely 90 minute exams (which are designed to cover a years worth of work) - I would say luck, would play a role conversely in the ATAR. It is not unsurprising to see even students with a very holistic learning approach often loose marks on nuance information that has slipped their revision nets whereas a lazier student with a smaller revision commitment can luck out on the day.
In the UMAT, since there is no crammable curriculum with a miniscule exam to assess it (realistically, less than half of what you learn will even show up in the VCAA exam for many of your science subjects) - the UMAT then becomes more of a technical game. Sure, there are the probability margins of the multiple choice questions, but that is miniscule.
Everyone does the same UMAT, with the same sections and the same chance of getting a question right in the event of a blind guess - the only element of luck here is that some people are lucky enough to be more gifted than others when it comes to naturally sitting the UMAT (and being so, the rightly deserve a better score) - that being said, with the right attitude, and proper technical approach, not to mention hard work and effort, anyone can get a good score in the UMAT.