I'm confused. What testing was used for the first child? Was it an actual IQ test? Or year end achievement testing? While a 99th percentile on achievement testing is a great accomplishment, it's not measuring the same things as an IQ test. I'm not saying this is so, but you could have two bright kids, with one performing better.
Does your school offer services that are worth testing for in 6th grade? If so, then I would probably persue further testing. If it's just for curiosity, you might consider full blown IQ testing for both children.
The test used was a modified IQ test - as far as I understand it. It's based on Cartell Horn Carrell theory (which I hear is the way things are going). It had 3 components (well 7 but they only used 3 in this version) crystallized knowledge, fluid reasoning and visual processing. I asked them how it translated into an IQ score and they explained that people aren't using IQ scores any more (this in spite of the fact that it's all I see everywhere on the internet). In any case it is based on the same elements as IQ tests. I hope that helps. I'm not sure if the previous test (the one my older son took) was an IQ based test or an achievement test. I know that it had a writing component which many agree can create a biased score.