This comes about because the part of their brains which deals with abstract thought (the dorsal part) are underdeveloped, but the parts dealing with language (the ventral part) are normally developed, because certain genes don't act during the formation of the brain.
What grabbed me, mathematically, was this:
These deficits generally erase about 35 points from whatever I.Q. the person would have inherited without the deletion. Since the average I.Q. is 100, this leaves most people with Williams with I.Q.’s in the 60s. Though some can hold simple jobs, they require assistance managing their lives.
Does this mean anything? Obviously "I. Q. points" are not something which just sits there in our brain. If my IQ is, say, 130, there aren't 130 little blobs sitting there in my brain which do my thinking for me -- or even 1.3 times as many such little blobs as the average person. (I know, you might be thinking that neurons are such "little blobs", but brain size isn't correlated very well with intelligence.) Furthermore, IQ scores are set up to have a Gaussian distribution, which I suspect is not the right thing to do. Perhaps the intelligence of individuals whose brains have developed "normally" are normally distributed, but the fact that there are a large number of disorders which "take away" IQ points makes me think there'd be a bump around, say, 60 or 70 IQ points -- a couple standard deviations below the median.
And that's only if intelligence is normally distributed to begin with. You'd expect that if intelligence were the sum of a bunch of independent effects, but I suspect there's some sort of synergy going on where "the whole is greater than the sum of its parts" -- a moderately above-average ability in, say, spatial reasoning and computational ability might make a better mathematician than someone who's really good at one of those and only average at the other. In general there are lots of complex skills which are made up of simpler skills in this way.
I suspect the central part of the distribution is approximately normal, though; the weirdness probably goes on with the very smart or the very stupid.