Our district wants both high ability and high achieving kids in their "magnet" for highly gifted. So they have to have a minimum composite score on the CogAT (or an IQ test) of 98th percentile AND achievement test results above the 98th percentile UNLESS IQ is over the 99.5th percentile or something in which case achievement testing doesn't matter.
They claim that there is a large percentage of quirky and/or 2e kids in the program, many with behavioral issues or various immaturities. It doesn't sound like the kids they are getting with their cut-offs are the perfect high-achieving ones. I'm a little surprised by that, given that in most cases they want to see very high achievement scores in both math and reading, along with the high ability scores. To get the achievement scores that they are looking for a kid would need to be doing above grade level work. For instance there would be no way for a third grader to get math achievement scores at the 98th percentile unless they learned their multiplication, division, fractions, etc. ahead of schedule. And someone has to teach them that.