There is no weighting based on position of the star in the frame. Every star has an equal chance of being used during focus and we use stars from across the entire frame to create an average HFR for the entire frame.
This gets you in best focus across the entire frame. I can't really see how pulling out the center would change this. This is even illustrated in your example above when you look at the mean for the frame.
One feature that were working on is to allow you to specify a subframe for auto focus. In your case you could specify a subframe that was a top or bottom slice omitting the center and very edge if this is what you feel is necessary.
Thanks
Jared