Why one shell can disable a ship but many are needed to sink it

Why can one or two shells cripple a warship while many more hits are needed to sink it?

Critical systems are vulnerable to a lucky hit, whereas sinking requires broad structural and buoyancy damage.

A single shell can strike a critical system and produce a mission kill, but sinking demands something different. A lucky hit may disable propulsion, weapons, command facilities, or another function essential to fighting. Destroying buoyancy requires damage across enough of the hull and watertight structure to admit water on a much larger scale. The contrast explains why hit counts alone say little unless the location and effect of each hit are known.