Nelson's stern flow made a Bismarck-style rudder hit unusually difficult

Why would a Bismarck-style torpedo hit on Nelson's rudder require unusual luck?

The Nelson hull's stern shape and water flow made the rudder a more difficult torpedo target.

Here, luck concerns the geometry and hydrodynamics of the Nelson design rather than a special torpedo defense. Water flow around the stern made a direct rudder hit harder to achieve than the famous strike on Bismarck. The point is probabilistic, not a claim that Nelson could not be disabled. Apparently similar torpedo attacks can produce different outcomes because hull form guides weapons and water around the target differently.