Why smaller torpedoes could cripple Blucher

How did a 120-kilogram torpedo warhead defeat a system rated against 250 kilograms?

The initial explanation is that the hit fell outside the protected area; the later follow-up qualifies the geometry and describes the combined torpedo, gunfire, and fire damage.

The question contrasts an apparent 250-kilogram torpedo-defense rating for the Admiral Hipper class with the 120-kilogram TNT warhead of the Whitehead Model 5D torpedo that struck Blucher. The initial answer attributes the damage to a hit outside the area covered by that protection. It adds that the vulnerable position would normally be at less risk while the ship maneuvered at sea. The later follow-up qualifies the geometry and explains the combined effects of gunfire, torpedoes, and fire, so the warhead comparison alone does not establish why Blucher sank.