By the way, I would not have developed a personality disorder by the age of three. Personality disorders begin in the teen years and adulthood. They don't develop in little kids who haven't even entered school yet (I've had autistic traits since I was an infant) and come from stable, loving homes.
I think the keyword there is "understanding." It implies a neurological cause. Why would some young children just spontaneously not understand how to interact with other people? If they were raised by wolves, maybe. I remember entering kindergarten and not understanding how to interact with the other kids. I was confused - I wasn't sad or depressed or anxious in any way. Not in the slightest. I just wasn't sure what to do. I still don't. I also preferred to wander off and play by myself. And again, I'm still this way, and I don't at all consider it a psychological problem--it is simply the way I am, the way I have always been.
I have rocked since I was a baby. Little babies in their cribs don't develop psychological disorders (i.e. there is no psychological explanation for my rocking). Other repetitive behaviors (insistence on sameness, adherence to ritual, intense fixated interests, etc.) also develop in toddlerhood (kids ages 1 - 3 years). Again, kids that young don't develop psychological disorders. Behavior observed that young in children belies a neurological/biological/genetic cause.