Your post is a little hard to understand as clearly as it is to you, but I am sure you knew it would be hard for us to totally get what you mean. Here's my take.
I get the feeling that when you say your are concentrating on having a solid position, that you are looking for a static place to find, and to kind of just freeze there . What you see in her is a greater fluidity in her position or aids, as is appropriate for the situation.
You are at the place where you are trying to be the best passenger that you can be. You are trying to make yourself as easy to carry and as little intrusive to your horse's movement as you can.
She is a driver. She has already been the passenger and doesn't have to think so much about that, it comes natural. So now, she is able to keep the harmony with the horse and still "drive" him
When we first start to ride, we spend a long time becoming able to follow the horse, in our body, with our hands, and with our minds. Over time, we learn to get the horse so follow us. We bring the horse to our motion, not so much the other way around.
So, I guess that she is using the rein and changes in her seat to ask the hores to come to her.
If you don't have a good solid seat, you cannot be quiet enough to have you message get across, like through the rein. So, perhaps you are where you are supposed to be at this time. Does your teacher feel you are progressing on track?