I've been following this thread. I don't know the Olsat so I looked around on Dr Google to find it is a multiple choice, group administered test. Reading your original question Nutmeg, I can see lots of reasons why this sort of test isn't necessarily giving you a score consistent with what you see in your child. Compared to having a child work with an individual assessor, you have no feedback on how your child approached the test, whether they understood or heard the verbally administered components, whether they were tired, hungry or distracted etc. Multiple choice tests are difficult for deep thinking kids who can argue the answers with themselves which can mean they lose time. They can talk themselves out of the right answer and they can fall behind in the verbal sections because they are still thinking of the question before.
With any test, if the results are not what you expect, you ask; was it the right/best test for what I want to know. If not, then investigate a better test. Then weigh the pros and cons of doing the test. Cost seems to be a major downside with individualised assessment and the time and potential stress involved. From my reading and personal experience, an individualised assessment with the WISC or, in our case the Stanford Binet 5 (minimal time stress), with an assessor experienced in giftedness provides a much better picture than a group test. So the short answer in my humble opinion is, if the results don't match what you expect, it is helpful and quite reasonable to go ahead and look at another test.
I would be very curious to hear how you get on if you decide to go ahead!
