I think there are perhaps some kids who are physically advanced because they are simply more athletic. Then, there are kids who are physically advanced when they want to be, simply because they are more driven. And of course there are kids who are very highly gifted and who aren't interested in physical stuff as much or who have other issues that hold them back.
Wrt the neck control thing, I think that it shows awareness and focus. It might also show early motor development, or it could be that most newborns are physically capable of lifting their heads but don't because they don't have the desire.
It is so hard to tell what is going on inside those little heads sometimes. DD is one of those kids who always seems to have more going on under the surface than we can see. When she was a toddler, before she was even 18 months, she regularly walked up & down stairs, alternating feet, without holding on to anything (I hovered with my hands out like a lunatic, but she never fell). Then she stopped, and didn't do it again until she was 4. We've seen this pattern again and again with both mental and physical skills, but mostly with the physical ones. If it was simply about ability, I wouldn't expect to see regression. So I think it is largely about drive. The kid likes a challenge.
