I might not be the most qualified person to say this but as a student who has done both Methods and Specialist myself I can say that they take very different set of skills to excel in, though they might have some form of similarity with one another.
Both methods and spec require attention to detail, and if you can avoid careless mistakes or be mindful of what you are doing at that point in time, you are basically set for a 35+, provided that you have learnt the course.
Through personal experience I would say that Specialist has way more content compared to Methods, which might throw some people off due to the content weight. (which is why they don't do as well in spec compared to methods) This means that to be able to assess every dot point on the study design VCAA has to write spec questions in a way that everything only scratches the surface of what we are required to know as students doing spec, and you hardly ever get questions that dive too deep because of time constraints and all that fluff. Students who aren't really careful with their calculations might score well because the questions are relatively straightforward, which leads to a better study score.
Methods however, can be written in a way that REALLY assesses your method of thinking, and ability to do questions because its contents are much easier to grasp. Rudimentary calculations in methods are plenty, and that can really hurt if you are more of a content learner compared to someone that can calculate well.