I also have three years of cogat data on my child and his scores have changed way more significantly than your daughters. His cognitive abilities don't seem to have changed and his school performance has been similar across these three years. I suspect that the different forms of the test are measuring slightly different abilities and that test conditions and child motivation on the day of the test also play a part.
My son's weakest area is language arts (though still above average) and his grades in that subject are excellent. I suspect that it just doesn't require 95th percentile+ abilities to get an "A" (or a "4" in your child's grading system) in language arts in elementary school. Perhaps I will see a dip in his grades once pre-ap and AP language arts begin in middle school and high school.
