Why is deck armour usually thinner than belt armour?

Why does deck armour often look thin compared with the belt, especially once aircraft become a threat?

The deck covers a much larger area and its weight sits high in the hull, so each added inch creates a greater displacement and stability penalty than the same local increase along the belt.

A ship's deck spans far more area than its vertical belt, making uniform thickness expensive in total weight. That mass also sits high in the hull, where it imposes a larger stability penalty than armour distributed down the ship's side. Designers therefore cannot compare deck and belt inches without considering location and area. Long-range shells also meet the deck at a shallow angle, so effective protection depends on geometry as well as nominal plate thickness. Aircraft increase the importance of overhead protection, but they do not remove the underlying weight and stability constraints.