With those kinds off cutoffs to qualify for extended norms, a single question could make a huge difference in score. That is not a robust system.
Really it would have been better if they'd tried harder to avoid ceiling effects in the first place.
The impression that I got from reading about it obsessively back when we first got DD's results is that the WISC was never intended as a tool for recognizing the gifted but rather as a tool for diagnosing issues at the other tail. Consequently, its ceilings are too low and things get too 'compressed' at the tip of the RHS tail.
