We had something similar with the WISC. Dd#2 was tested twice on the WISC-IV a year apart. At the end of 2nd grade, her IQ was in the 99.9th percentile and at the end of 3rd, in the 97th. It may not seem like a huge drop, but it was a 20 pt drop primarily b/c her perceptual reasoning score went from the 99th to the 75th percentile. The first time around, her raw scores on that index were 18s and one around 15 or 14 (not right in front of me). The second time, she didn't have a single raw score over 12 on that index.
According to dd's school, the PR index correlates best with math. I attribute her drop in that area not to an actual drop in ability, but likely due to her having really lost confidence in herself in 3rd grade. Her achievement scores in math were in the 90s in 2nd and had dropped to the 50s and 60s in 3rd as well and her teacher kept telling us that math was not something she was good at. Her prior year's teacher felt that she was very good at math and so she was that year.
Anxiety, self-doubt, and people who don't believe in her make a huge difference for our dd in terms of how she tests. Unfortunately for her, acceleration decisions re math were made in 3rd grade, so she will never qualify for math acceleration based on those lower 3rd grade scores. As long as she remains in this district, she will never be able to accelerate in math. Fortunately, she will be changing districts in middle school. Our hope is that, if she can start performing in math again, we can revisit her math placement with the new district in 6th grade (she's a 4th grader now).
