Preferred Name Populates on Legal Name Change with First Name, Why?

We are implementing preferred name and found that the field is blank but if you go to the page to do a legal name change or even just to view the page, when you save or when you try to cancel out of the page, it is populating the preferred name field. It looks like nothing is in the preferred name field, but it is automatically populating the employees first name in the preferred name field, why? We do not have required check on the "Name Format Table", yet it is still populating the field. Any help