When some highly qualified researchers from universities compared Daniel Tammet with a man who appears to more closely conform to the established concept of the savant, a language savant named Christopher who has definite disability, Tammet's performance was quite different in at least two different ways. Who'd have thought? As far as I can tell from his own account of the testing that he did with two of the authors of the book below, Tammet might not have known that his performance in one of the tests would later be compared with that of a(nother) savant.
Neil Smith, Ianthi Tsimpli, Gary Morgan, Bencie Woll
The Signs of a Savant: Language Against the Odds. Cambridge University Press, 2011.
See pages 167 and 221 in this book for Tammet's account of this research:
Born on a blue day: inside the extraordinary mind of an autistic savant : a memoir. Simon and Schuster, 2007.