The 3" bars provide three of these offsets, so that they match the base widths of the most common mounting rings. Nevertheless, the bearing surface is hyperbolic on the telescope side, so that the bar will fit tightly for all telescope diameters between 160 and 600mm.
All three bars have got a central, 10.5mm wide pit on the upper and lower side. Here a screw head with up to 10mm wide fits into it, so that the head's surface is not visible above or below the bar (Only exception: The 3" bar is flatter on the upper side, so that a screw's head would poke out of the central pit. This was done to keep the bar's profile as flat as possible, for a short distance between telescope and mount.