Illustrious-class protection was intended to defeat a 500-pound or 250-kilogram bomb and give useful resistance to a 1,000-pound or 500-kilogram weapon, especially when a pilot released low enough to reduce impact velocity. A roughly 1,000-kilogram bomb placed far more mass behind the strike and exceeded that design basis. The semi-armour-piercing version used a thick casing, sacrificing some explosive filling so the charge could detonate farther inside the ship. Such a weapon made particular sense against armoured carriers even while Axis attention and propaganda focused on the unarmoured Ark Royal. Nominal bomb weight, explosive yield, impact velocity, and penetration all affected the threat.
Notable Point · 1:09:52-1:13:44
Why one-ton bombs changed the armour calculation
The Illustrious class was protected against smaller bombs, but a German one-ton weapon brought enough mass and penetration to exceed the flight deck's design basis.