This all can be explained by how the brain develops in the autistic. What you are describing is one of the hallmarks of autism,..."asymmetrical intelligence". How can a person have a photographic memory or be a savant in a particular area, yet have significant difficulties with the basics of daily living activities?
If you have studied the research on autism as it pertains to neuro-imaging, then things become quite apparent. Regardless of the underlying causes,...and this goes well beyond this discussion,...what we can see is that there is a list of things going on in the autistic brain: (1) As cells migrate from the center of the brain outward into the cortex, there is supposed to be an organized layering effect,...and in some autistic phenotypes it can be quite disorganized. (2) As cells migrate, there is also supposed to be organized functional areas,...and in some autistic phenotypes, this can be disorganized,...leading to connections where there shouldn't be,...leading to side effects like smelling color, or numbers having color, tinnitus, visual snow, tactile issues, physical coordination issues, tasting temperature,...a long list of "odd ball" sensory experiences. (3) The normal pruning of synaptic connections that occurs before the age of 5, may not happen in all the functional areas of the brain, leading to asymmetrical conduction and connection issues,...(thick wires and thin wires, some with high voltage and some with low voltage). It can lead to neurotransmitter imbalances in areas where there are "thin wires and high voltage",...and areas where there are "thick wires and low voltage", as the biochemical production of excitatory and inhibitory neurotransmitters and their metabolic co-factors may be out of balance. This is, in part, why it is very difficult to properly dial in medications,...what may help in certain areas of the brain, may create a worsening condition in another area of the brain. It also can lead to much of the "mental exhaustion" that many of us experience. It can lead to repetitive behaviors, stimming, etc. It can lead to thalamo-cortico dysrhythmias,...common ones being tinnitus and visual snow,...an inability to hold your hands steady (constant, low-level tremor), and so on. It can also lead to asymmetrical intelligences,...high-level pattern recognition, visual thinking, mathematics, music, and so on,...but have a difficult time with their daily activities.
I am sure there is much, much more to all of this, but to in order to understand how an autistic person can score very high on certain areas, and not in others,...is due to how the brain has developed.