Developmental differences can be either cognitive, physical, or both. So say at an age of six years, it would be normal for one child to be physically three years ahead of another child of the same age. That same child could also be three years behind in cognitive development. This would still be normal. Experience also shows that physical education seems to make a difference.
Physical education teaches children how abstract concepts relate to the physical world.
The quality of a child's physical education becomes obvious during a ski lesson. A child may grasp the concept of up and down, but may not understand the concept when it becomes more abstract. Every four year old knows dogs bite, not skis. Physical education is important because it teaches young minds how to process these kinds of abstract ideas. Just in case you're wondering how top-notch skiers get started, here are some rough benchmarks.