The primary reason that the range for average was changed for WIAT-III was to align with other tests of achievement. These kinds of tests typically use one SD from the mean as the definition of average which incorporates around 68% of the normal curve. Ability tests, on the other hand, typically use the standard score range of 90-110 which incorporates around 50% of the normal curve. Neither is incorrect, and use is a matter of convention. Practitioners are free to use whichever definition of average they find most useful.
