I have a close friend at work who has 5 year old TWIN boys. One is autistic, the other is not.
Saw a documentary the other night at my parent’s about the identical twin girls (now in their 50’s) that are autistic. They have amazing memories and are obsessed with Dick Clark. Interesting stuff.
Even identical twins are not verbatim molecularly identical, and virtually always have different personalities. Brain chemistry is associated with many sorts of chemical cycles that are kept in various states of metabolic "balance." It wouldn't take much of a flaw to disrupt, for example, sensitivity to a process that monitored such a metabolic balance, which would then have a dramatic effect on brain function in a way that couldn't be easily corrected. Metabolic triggers are often extremely minute - an example is the crucial importance of "trace" minerals. So, with something as potentially toxic as mercury, it wouldn't take much of an increased sensitivity to have a very different reaction to identical "trace" amounts between children.
And when they are young enough to still have developing immune systems (such as when they first start receiving massively combined vaccinations nowadays), it is not at all inconceivable that the vaccinations could overwhelm the immune systems of the children who have not yet developed as much as others.
Removing squalene and mercury and other known toxic materials, and pacing the vaccinations over a longer period of time and separating them so the immune system is stressed as little as possible, would be easily available solutions to huge concerns about vaccinations.