My children haven't taken the SB-5, so I don't have any experience with it, but I suspect there are multiple reasons you might see such a difference. Timed vs untimed isn't a large factor here, because you've taken the timed processing speed subtest scores out of the WISC when you are only looking at GAI. Do you have your dd's WISC subtest scores? It might be interesting to know what her FSIQ was and how her WM vs PSI scores varied. If I remember correctly, there's at most only one other timed subtest on the WISC.
The two tests are different, and I've always heard that the SB-5 is a better test for students who are (I think - I could be remembering this incorrectly) - stronger in non-verbal and visual-spatial etc areas.
It's possible you're seeing ceiling issues on the WISC - do you know if extended norms were used in the calculations? Did your dd have 18s and 19s for subtest scores? I think the SB-5 doesn't have the same ceiling issues. I know that when our EG ds was tested by our school, they preferred to use WJ-III Cognitive scores than WISC and one of the reasons was ceilings on the WISC possibly underestimating IQ.
As a mathematician, I suspect you also may be running up into territory where simply because your dd is at the upper end of the bell curve the curves don't fit well and perhaps we really just don't have the data to be able to say that score X on the WISC is equivalent to score Y on the SB-5 when you are up above the 99.9th percentile.
Last but not least, even scores on the same test can vary. My EG ds has taken the WISC twice, same tester, three years apart, and his scores on VIQ vs PRI totally flip-flopped in magnitude between those two tests. His overall GAI came out about the same value each time... because it was an "even" flip-flop. But why did they flip? Was it the amount of sun coming through the window on test day? Difference in performance for different ages in the norm group? The inflection in the tester's voice tone? I am fairly confident that his abilities have stayed similar through the years and he didn't all of a sudden morph from a verbal kid to a mathy kid or whatever... I think it's more a case of understanding the scores aren't exact, they have error bars around them.
So that's my 2 cents, fwiw!
Best wishes,
polarbear
