As I understand it, the SB5 has sub-test sections in batches. With the WISC, things can discontinue fairly snappily. If your kid has some perfectionism (which can be a bit innate or just the result of never being challenged by tests) then hitting his level early in a batch could mean he is facing like four or five more questions beyond his abilities which can be pretty stressful. A smart kid may also know when they are wrong without knowing what the right answer is.