This happens in my DD's gifted magnet as well--some kids struggle terribly with the math, others with the writing. IQ of 130+ actually still does not sort all that well, surprisingly (at least to me). It points to the important of finer-grain differentiation, which may or may not be achieved through tracking and grouping. I actually think there is no grouping specifically because parents would freak out over it and protest placement in the lower group.