I hear all the time that PG kids are underserved in most schools and that parents need to advocate aggressively to get the most basic of accommodations - and those are the parents who know how to navigate the system. There are also parents who have no awareness nor the ability to advocate due to various reasons.
So, if a kid with IQ of 150+ does not get any acceleration or differentiation and has never seen advanced topics in math, how would that child work 2-3 grade levels ahead in that subject? Intuitive understanding due to high intellect can not take that child too far. I believe that the correlation between high IQ and working at a higher grade level is not a given. It is mostly due to the learning opportunities available to the child.