A few thoughts on pace of acceleration and it's interaction with age:
In the early years, almost all of reading and mathematics consist of basic skills, with minimal abstract thinking. A child with exceptional memory can acquire these skills very quickly. A child who has exceptional abstract thinking, but -not- exceptional memory might be a little slower to pick these up, but as soon as s/he reaches mastery of the basic skills, s/he will surge forward.
For writing, there are additional factors having to do with fine motor skills.
In what primary/elementary teachers call the content areas (science and social studies), basic skills are minimally relevant, except insofar as they restrict access to written sources of information. Language comprehension, social comprehension, and conceptual grasp of the scientific method are more important.
Oh, and I think we all realize that there is no new math throughout elementary school, beyond the four basic operations (two of which are derived from the others). So once you've mastered those, it is quite easy to blaze through several years of math in a year.
I think that part of the spurts of growth that one sees in gifted children is a function of the switch from basic skills to higher-level reasoning, problem-solving, comprehension, and expression in middle/late-elementary level curricula. There are multiple paths to basic calculation and decoding skills (especially reading), but once you pass that hump, abstract reasoning takes over.
And, BTW, HK, I have a sibling whose actual acceleration profile was very similar to your DD's idealized profile. My mother was concerned about the huge gap in writing skills (translation: zero note-taking in college), but found that my sib's memory and comprehension were such that notes were unnecessary. The other adjustment was that the required English composition class, history/geography, etc. waited until the last term before graduation. Of course, this required a fair amount of flexibility on the part of the university, which happened mainly because there was faculty support.
