9-patch image files are confusing. Heres an article that clarifies what it is and how it works

One use of 9-patch files is for scaling buttons. With a 9-patch file, you can specify which parts of the button will be stretched (or squished). With a non-9-patch image file, you can only do uniform scaling and the button will look weird. For example, for a button that is stretched only length-wise, you don’‘t normally want to scale the four rounded corners of a button. This is because a uniform scaling will also stretch those corners and it wont look right..