It’s the downward bevel angle. I think it’s 12 degrees. Go to your table and put the balls anywhere you like within the jaws. Then lift them straight up. They will get closer together.If the points are the outer most part of the rail, how would the balls touch an area inward from there first?
4.5" should be 4.5" regardless if it's across the equators of two spheres measuring 2.25" in diameter or across the span of a 4.5" measuring device.
The balls touch at the facing well below the visible points in a picture due to this.