1. I'd say that it has less to do with an improvement in mathematical ability, than it does exam experience, maturity and composure. The way I see it for Methods, is that the content isn't actually too bad. Everyone can learn to do the basic formulas, familiarise themselves with the material and 'understand' everything about the course per se, but those who push for the higher scores are those who can perform in an exam and have their knowledge remain firm against VCAA's inevitable "unseen" questions.
2. imo spesh is a bit more plain in terms of its imagination. The maths itself is definitely harder, however I do agree that it requires you to think less outside the box and establishes its difficulty based on the math alone - not some scenario trickery as is often seen in methods. To put it in other words, methods I would look at a question and be able to visualise every single step and operation I would need to complete to reach a certain answer, but in spesh I would often look at a question and have to put pen to paper, crunch numbers and run some calculations before the answer would appear.