I like the limiting factors idea (thanks ColinsMum!) -- it would explain DS much better than any straight measure of bad-at-things to good-at-things. He walked very VERY late... limited, I'm guessing, by a tendency to extreme caution. He talked late, and we found out long after that phase that he had a structural defect (tongue-tie) that was a likely limiting factor.
I think where all these things show up is that once he had mastered the part that was holding him back, his development was explosive. And the predictive value of the early milestones was virtually nil... the late walker can tap dance beautifully. And on the talking side although his articulation isn't great, his vocabulary and sentence structure is. (Bad combination, by the way...)
He still has "artifacts" of his early development (still physically cautious, and still drops sounds when talking), but they don't match up with the rest of his ability... and my bet is that they never really did.
