This thing is a very clever design. Since the Yamaha axle is a larger diameter, it had to be bigger on both ends than in the middle,and trust me: That would make mounting the wheel quite difficult!
So JDRadman's brilliant solution was to put a nut at one end of the axle, that is also rounded and acts as the bearing surface for mounting in the fork leg.
They say a picture is worth a thousand words, so here's an essay:
