Why Does A Child Sway Her Head Constantly?

Our nearly 6 month old, completely healthy and happy granddaughter sways her head constantly when on her tummy or is being held. It is not because she is sleepy. At first we thought it was because her head / neck muscles were not strong enough, or because she was swaying to the music. She is sitting up fine,alone, though she still has to concentrate on sitting up. Her head is still when she sits up, perhaps because she is concentrating on her balance. One family thought is that perhaps her eyes don t move, so she moves her head to see! The baby s mother will ask at the 6 month check up in a week +, but we are interested in what you have to say.