As a second grade teacher for the past 20 years, I have administered the Comprehensive Tests of Basic Skills exam to hundreds of children. There are many reasons that test scores often do not reflect the child's true ability.
Some excellent students receive low scores because they daydream, need to use the bathroom, or get distracted by a broken pencil. Some weak students receive high scores because they "guess lucky." I have seen children receive scores in the average range by simply marking the first bubble in each question. I have also seen children finish a 20-minute section in 20-seconds without reading a single question. Others who are very capable, but work slowly and carefully, fail to finish in the time allotted and are penalized. Some, who finished quickly and accurately, have changed right answers to wrong ones out of boredom while waiting for the others to finish.