Ghast's Fireball's blast

I encountered an issue with Ghast's Fireball during my gameplay.
On Minecraft wiki, it is said that "The minimum blast resistance required to absorb all blast force of the ghast's fireballs is 20.17 (about the equivalent of a cobweb)." (Source: https://minecraft.gamepedia.com/Ghast)
Therefore, the most common way to block/reduce Ghast's explosion is using Cobblestone, Stone, Stone Bricks, etc... And according to this: https://minecraft.gamepedia.com/Explosion#Blast_resistance, Stone Slab has blast resistance of 30. BUT, when I use Stone, Cobblestone, etc... Sure, it can block Ghast's fireball, but none of the slabs like Stone Slab, Cobblestone Slab, Nether Brick Slab, Stone Brick Slab can "absorb" the explosion. Is there something about slab that I don't know?