A couple of ideas. I'm not a physiotherapist per se, but I've been involved in enough different types of physical activity that I observe certain things...
1) Everybody's anatomical and physiological layouts are just slightly different, even though fundamentally the basic components are the same.
2) When you really think about it, are you really relaxed when you play a challenging videogame, rather than a relaxing one? Various parts of your body might tense just a little bit, bracing for a period of fierce button-mashing or stick-twirling. The more I think about it, the more the medial angle of those buttons makes sense. I'm not sure I would use that design myself, since I plan to use a mild, symmetrical curve. But I do see it as usable.