Two things come to mind. First - is it a "fill in the bubble' test? Maybe he somehow got "off" the correct row.
Secondly, maybe he was just having a really off day. One of my sons (age 7), who clearly demonstrated addition fluency during first grade with timed tests, completely blew off that section of the WIAT-III and received a very, very low score for that section. All of the other sections are in the high 90s for percentiles. There is no rational explanation, because he KNOWS his addition facts and passed 26 similar timed tests in school this year very easily. His only explanation is that he has been thinking about "subtraction" a lot lately. Luckily, this doesn't really matter as far as school goes, but I could see how horrible it would be if we were counting on this test to get into a gifted program.
