I'm not sure about the variation other than to say that IQ scores aren't stable in kids until they are around 8 (or some would say even later, like 4th or 5th grade). The 5th grade score probably has the highest probability of being accurate, unless you think you scored low(er) on purpose. Obviously 118 is still a good score. I also don't know anything about that particular test and the margin of error. Maybe aeh (the test expert here) can tell you something.