I like the WISC-V better than the WISC-IV, actually, and generally find that it is better for 2e kids. There are, however, several factors that I can think of off the top of my head, not including the possible long-term score depression effects of unremediated disability.
1. Norm obsolescence: He was tested very late in the norm age of the WISC-IV, and quite early in the norm age of the WISC-V. That is good for a drop of 3 points for average cognition kiddos, and quite possibly more for children in the upper extremes (where the Flynn effect appears to be more pronounced, and less predictable).
2. GAI factor weighting: The WISC-IV GAI is roughly one-half verbal, one-sixth visual-spatial, and one-third fluid reasoning. The WISC-V GAI is two-fifths verbal, one-fifth visual-spatial, and two-fifths fluid reasoning. A child who is strongly verbal, but not as exceptional in fluid reasoning, may experience a drop in GAI, as his area of strength is a slightly smaller contributor, and his area of relative weakness is a bigger contributor.
3. Developmental trajectory: the level of abstraction required for scoring far to the right on the curve increases steeply after latency age (around age ten or so, entering the tweener years), as the population shifts toward abstract thinking. If one is ahead by the same (hypothetical) absolute amount in abstract thinking at age 11 as one was at age 8, it's a smaller distance to the mean at age 11, because hardly any 8 year-olds had abstraction, but a great many 11 yos, do, at some level. In addition, LD kids not uncommonly have unusual challenges at certain developmental transitions.
