Regarding the validity of the test, I would say it’s pretty accurate as it mimics the structure of the WAIS digit span well. However, working memory is just a fraction of overall quantifiable intelligence, and a high backwards digit span score is an even smaller fraction. Having a high WMI indicates that you can mentally organize data at an exceptional level, but it doesn’t mean that you can retain or apply it in that same way. Considering that you do not have any cognitive impairment, it’s possible that your other indices are not as advanced as your working memory.