OK - I'm going to stop trying to explain myself here, because I'm making a mess of it.

But to the OP - when our tester used extended norms, my DS's FSIQ went up by almost one standard deviation (14 points) and his VCI went up by more than two standard deviations (33 points). So I'm darned glad we have that data. When it makes a difference it can make a huge difference. I'd want to know they would be prepared to provide this for your child if, as George summarizes, it's appropriate to calculate them.