Ugh, file this one under "Bad Linguistics." Daniel Tammet, the autistic savant whose astonishing ability to calculate and recall numbers was documented by the film The Boy with the Amazing Brain, has been utterly unimpressive when it comes to language. Despite his claim to have learnt Icelandic in a week, he gave an absolutely underwhelming performance in the documentary. In fact, his level of Icelandic after a week's study was about on par with any normal adult that has gone abroad and hit the books every day for seven days.
I was also less than thrilled to hear the documentary talk up Icelandic as if it is some sort of holy grail of language learning. Oh, please. It's a Germanic tongue that separated from English several thousand years ago. It's closer to Norwegian than to English, but it's fairly close. Not to mention, English-speakers endured several hundred years of Viking invasions - invasions that left a profoundly Old Norse impact upon our speech (Old Norse was the ancestral tongue of Norwegian, Swedish, Icelandic, etc...).
Well Tammet's come back with another Icelandic screwball. Tammet writes an article on the declination of adjectives in Icelandic that betrays a clear ignorance of grammatical gender. In Icelandic, the first four numbers (when used as adjectives) are declined according to the noun's class. That leaves each Icelandic numeral with three possible forms. This is the product of language evolution from Proto-Germanic; just as Old Norse is the ancestor of Icelandic, Proto-Germanic is the ancestor of Old Norse, and if you think Icelandic's system is tough, take a gander at the declension of number one in Proto-Germanic (make sure you click 'Declension of *ainaz' to unhide it).
Tammet also fails to recognize that we have plenty of numeral alternatives as well. Is a separated group wrest in two or wrest in twain? Did you celebrate the four of July or the fourth? Did the song go "One time, two times, three time's a lady?" The fact that Tammet missed this obvious fact of English has me confused thrice-over.
But all this is fairly moot. The declension of a noun according to its grammatical function is a normal process of agglutinating languages and Icelandic is a moderately agglutinating tongue. This does not arise due to a "special" psychological process, as Tammet suggests, but out of normal language evolution and the maintenance of a case system. A case system is quite normal. English used to have one, and many languages actually embiggen their case systems over time, as in the cases of Polish and Finnish.
The long story made short is that Tammet needs to go back to the books.
Thanks to /r/badlinguistics for pointing this article out.