.... because the nut tightens up against the crankshaft sleeve so the nut is basically a locknut holding the sleeve firmly on the crankshaft. There is a common misunderstanding that the nut should only be tightened sufficiently to compress the spring but actually the nut should be fully tightened, at which point the spring tension is correct. The nut has slots so it can be finally tightened with a chisel, I know this sounds disgusting but this method of shock-tightening is the most effective way of achieving a firm grip through the components which are intended to be locked together. There should also be a lock washer between the nut and the sleeve which is a bit difficult to peen over because you need to poke something through the spring to achieve this. There should be a hole for a split pin through the crankshaft - this is only as a bit of security to prevent the nut coming off completely in the event of it working loose. Some people think that once you have screwed the nut up enough to get the split pin in, job done, not so! The nut must be locked up against the crank sleeve. Hope that explains it.