Savant syndrome is rare, and I haven't studied it much, but I will just put here my thoughts about it.
Some people think it is somehow puzzling that an individual can have such extraordinary ability in some obscure form of calculation or calendar-memorising or such-like. Especially if they are not particularly high IQ. I don't find it particularly suprising myself, at least not more so than the already amazing things that the brain of the average person is capable of anyway.
For instance if you have any experience of cheapo binoculars you will know that the human visual system has an ability to coordinate two misaligned images into one. And indeed it does this all the time even when binoculars are not involved and even when stereoscopics and other factors make the images disalike. This effortless merging of binocular images must take some rather substantial hardwired computing power.
A second less obvious example came to my attention after I was attacked by a thug and left for dead. A few weeks later I was amazed to find that my right ear was hearing sounds half a semitone sharper than my left ear. Which causes all music to sound very unmusical indeed. This problem, the technical name of which I have forgotten, resolved itself on the 13th day just before it had driven me completely bonkers. But again, it shows that the auditory brain must be likewise effortlessly mapping together the disparate sounds from the two ears, even while our attention is concentrated on other things such as understanding or appreciating the sounds in question.
These high-power computations are performed without effort in the brains of even the most average of people. Let us combine that fact with the observation that in normal development, some neurons have to "migrate" their axons and dendrites significant distances to establish connections with other parts of the brain or body. Under certain conditions it can be expected that such migrations will get misdirected, and this would in respect of sensory neurons quite credibly produce the mix-up of sensations that is synaesthesia.
Meanwhile there could occasionally be another sort of misdirected migration-connection, in which an area which would normally be innately-assigned to one of those complex innate functions such as binocular vision or hearing gets connected to some other inputs and outputs and thereby recruited for some other task instead. And would thereby produce, rarely, one or other savant syndrome ability.