My hypothesis (I e-mailed Simon Baron-Cohen and he agreed) is that autistic people don't subconsciously "synchronize" with people in the same way that neurotypicals synchronize with each other. "Synchronization" seems common in the animal kingdom - this is why spinner dolphins can be extremely good at coordinating their movements together without even having to communicate at all.

So autistic people have to learn these signals very consciously, as explained in all these other posts. And conscious learning is often clumsy (which often means oscillatory convergence around a fixed point)