Of course I understand that doing poorly on a spelling or mathematics test doesn't necessarily mean that the subject has reduced reasoning skills. An IQ test is not a simple spelling or mathematics test, however. Jessie's full range IQ was consistently in the 70 - 74 range. This range is classified as borderline mentally retarded which alone would not prove diminished reasoning ability, but it would certainly put up a big red flag that diminished reasoning ability might be present.
In order to be admitted into Special Education classes (which Jessie was), more is tested than the subject's IQ. Although IQ is usually the first thing tested, then tests are conducted to see if the low IQ score is a result of a learning disability such as dyslexia or dyscalcula. Once learning disabilities are ruled out, further testing is usually performed to more accurately determine the reasoning ability of the subject, or the lack of age-appropriate reasoning ability in the subject.
In Jessie's case, from what information has leaked out (since the school records are sealed), it is painfully obvious that he reasons at a very low level. Dr. Rickert didn't say that the low IQ scores ruled out a lower level of reasoning. He was simply pointing out that the IQ test alone is insufficient to determine the reasoning ability of the subject.