The explanation compares a thinner outer layer with a heavier inner belt rather than treating armour as one uniform wall. The outer hull can be used to initiate an explosive effect farther from the ship's interior, while the inner protection limits how far that damage travels. This is a design question about where a shell should be stopped, with no guarantee that every projectile will behave identically. Armour schemes depend on geometry, spacing, expected ammunition behaviour, and plate thickness.
Notable Point · 1:10:03-1:11:01
Layered belt armour and where shells detonate
A layered-armour thought experiment considers using an outer belt to trigger a shell before an inner belt protects vital spaces.