Yes, the scores are very inconsistent. So it sounds like both verbatim memory tasks were very strong (DSF, PS), as were both sequencing memory tasks, where cognition is in the mix (DSS, LNS). It's just DSB. Some individuals use visual memory to do digits backward; I sometimes find lower DSB accompanying visual relative weaknesses. This would not necessarily show up on picture span, as they are concrete-familiar objects, which one can convert to words, and then use verbal memory to store.
Any indication perceptual skills are affected?
Another possibility, especially with young children, is that he or she just did not grasp the task. It's not uncommon for kids to process "backwards" as say the last digit, then say all the remaining digits in presentation order. (So, given 1234, repeat them as 4123, instead of 4321. You can see how this works on the teaching items, which have only two digits, and on the early 2-digit sets, but not the minute you hit three digits.)
If you and the psych have reason to believe DS is not an accurate reflection of your child's true skills (say because of something like my example above), you can always ask to calculate the primary index scores and FSIQ using LNS instead of DS.
