This is my 3rd attempt at a reply!! Hopefully it will post this time:

from what I understand, "to calculate the General Ability Sum of Scaled Scores. The General Ability Sum of Scaled Scores is the sum of scaled scores for three Verbal Comprehension subtests (i.e., Vocabulary, Comprehension, and Similarities) and three Perceptual Reasoning subtests (i.e., Block Design, Matrix Reasoning, and Picture Concepts).

I can see the discrepancy in the PRI, she actually had 4 subtests in PRI:

Test/scaled score/%

Block design/15/95
Picture concepts/15/95
Matrix reasoning/19/99.9
Picture completion/12/75 (I think this was a substituted test)

Her Verbal Comprehension tests were:

similarities/16/98
vocabulary/14/91
information/19/99.9

The tester is one of the test question developers for the test, and knows the test very well. Not sure why he did substitutions. My daughter was 6 years 7 months when the test was given, she is 10 months older now.

I guess I was interested in the GAI because:

"The practitioner may wish to consider using the GAI in a number of clinical situations, not limited to, but including the following:
• a significant and unusual discrepancy exists between VCI and WMI;
• a significant and unusual discrepancy exists between PRI and PSI;
• a significant and unusual discrepancy exists between WMI and PSI; or
• significant and unusual intersubtest scatter exists within WMI and/or PSI."

Source (PDF): http://www.ksde.org/Portals/0/Special%20Education%20Services/gifted/WISCIVTechReport4.pdf

Still not sure how to calculate the GAI with the 4 subtests and I am not sure how he calculated the PRI..???