I'd suspect that the tester did something off unless you did. There really is not magic or subtlety to calculating the GAI. You just add up the scaled scores from all four indices and there it is.
In looking at the WISC technical report #4, though, a sum of scaled scores of 87 should give you a GAI of 132, which is the 98th percentile. The other thing I'm wondering on, though, is the FSIQ being 135 if the PSI index was significantly lower. I have one child with that type of profile and I am having a hard time imagining the FSIQ and GAI being identical in that instance. We saw something like a 7 point bump on the GAI over the FSIQ b/c PSI was average and the other three indices were in the upper 90s.
I totally understand if you don't want to post all scaled scores here, but overall something sounds a bit off.
