Why do people have different development trajectories?

I'm reading this fascinating new-ish article by Frankenhuis and Panchanathan--anthropologists publishing in a psychology journal--and the authors argue that we usually sample cues from the external world in much the same way that psychologists sample populations: randomly. With psychology research, you hope that you mostly get typical people in the "middle" of the population range with a few in each tail. But you keep in mind there's a chance that purely by coincidence, you've selected a sample that doesn't have that nice bell-shaped contour. (This is why larger samples are better; over a larger samples, the odds of this mishap decrease--but they never reduce to zero.) Similarly, when we, as biological beings, sample cues from the external world, the majority of us get a truly random sample of experiences, which consists of mostly typical stuff with a tiny tail of unusual stuff at each end. But a few of us get an awkward mix that doesn't make sense. Instead we get a heterogenous set, and so we have to keep sampling. Thus, the people who get the nice bell-shaped sample develop faster, while the others take more time.

The authors make three predictions about what data we should find if this hypothesis is true. But they don't have data, so this is conjecture, albeit very informed conjecture.