I'm going to go out on a limb and speculate that the class your DD is in had the higher test results in math. I would not be surprised, from your description, if the other class was more heavily weighted toward hot-housed children and those from influential families--i.e., not quite as high ability, intrinsically. Consequently, more even in academic achievement. Alternatively, this is the high math class, and the other is the high reading class, and those who were high in both were placed according to their higher score. Probably would have the same correlation postulated above.
As you say, a little like the composition of some selective colleges, actually: lots of privileged legacy-type students, and lots of top-tier academic foreign-born or first-generation immigrant students.
