Why armor-plate joints complicate deck protection

Why could a battleship with apparently substantial deck armor still be penetrated while another ship survived a direct hit?

Armor layout matters as well as total thickness, because plate joints can become weak points and staggered layers can prevent those gaps from lining up.

A single thick armor plate offers strong resistance, but an armored deck has to be assembled from many plates across a curved hull. The joints can create vulnerable paths for a projectile. Dividing the required thickness into overlapping layers makes the seams less likely to align, much like staggered bricks. Each layer offers less resistance on its own, while the overall arrangement better controls weakness at the joints.