One theory that does provide an increasingly plausible explanation for savant abilities in many cases is left brain injury with right brain compensation.
…
There is emerging evidence that prodigies and savants may share certain underlying mental processes when carrying out their specialized, expert tasks. Event-related potentials (ERPs) can measure very early
components of brain activity, reflecting initial, "pre-conscious" stages of mental processing. This fast, low level "preconscious" mental activity contrasts sharply with that seen when higher level, "executive" functions are accessed during typical information processing. Birbaumer compared ERPs of a "human calculator"— a non-autistic arithmetic whiz — to same age, IQ-matched, healthy controls.46 Compared to controls, early on in the calculating process, the expert calculator showed evidence of "enhanced automatic low-level processing." Studies are now underway with autistic savant calculators to see whether this particular type of early, lower level processing ("without reckoning") is the same as that used by the non-autistic "expert" calculator.