Belt extensions do not automatically need the thickness of the main armored belt. Projectiles arriving from ahead or astern are more likely to meet the plating obliquely. The bow and stern also usually sit outside the armored citadel that contains the ship's survival-critical spaces. Lighter extensions can therefore protect the hull ends without carrying the full weight of the main belt.
Notable Point · 1:24:35-1:25:02
Why extended belt armor can be thinner
Armor beyond the main belt protects the hull ends but can be thinner because end-on hits arrive at less favorable angles and those spaces sit outside the central survival zone.