The controlsWidth property defines the width on the background texture, for the controls bars.

In other words this means, that the coordinate is not in Widget space, but in texture space. If your background image has a width of 1000 and you size the Widget to a width of 250, a value of 500 means 500 * 250 / 1000 = 125 Pixel on the HUD.

This makes sure, that the controls bars are always the same size on the background image no matter what size your Widget has (the background image will always be scaled to the same size as the Widget (without border).