ORIGINAL: Fallschirmjager
[Yeah I know it was an modified 800 kg naval shell turned into an aircraft bomb. But did not the Navy in 1944 find in their report that it did not penetrate the magazine?
The 'bomb' was rated close to being able to penetrate Arizona's deck armor. There's wiggle room IMO. The item however of central focus in the official investigation IIRC via 'Battleship Arizona' was the black powder magazine located next to the main armament powder mags. The issue wasn't so much penetration as explosive content of the bomb which was only 4% of the bomb's weight and it was thought highly unlikely that such a small amount of explosive might have detonated the fairly 'stable' white powder used for the 14inch guns.
However that was not the case with the black powder used for the ship's catapult. It was highly explosive. The key point focused on was the 7 seconds that passed between the ship being hit (and a small internal fire noted) and the magazine explosion. It was felt that even if the bomb did not penetrate, that being a Sunday, and being at peacetime, that the hatch to the powder mag was possibly open at the time. The "interval" between the hit and the explosion might be explained thus by fire moving from the bomb impact, through the hatch and into the black powder mag. The BP mag goes off...which in turn touched off the main gun powder mags.
Thats I believe the most prevalent theory. Of course the sheer level of destruction destroyed most of the physical evidence so we'll never be 100% certain.