My experience with both my sons is that the scores jump around a fair amount. The school tests them every month or so in the Fall, and each child's most recent score determines the difficulty level at the beginning of the next test. So if a child scores really high in September, the October test will be hard. For my boys, after they hit 12.9, the next test would inevitably be lower.
By the same token, if your son had a poor result on this test, the next time he takes the test it will be easier and he will likely score higher.
That said, the swings for my boys were always within a few grade levels.
