Wooden Slab's Block Type To Stone

The Wooden Slab is unable to be broken by an axe, but is able to be broken by a pickaxe. This is pretty weird since it is a type of wood.
Steps to Reproduce:
1. Place down a Wooden Slab
2. Destroy it with an Axe (It won't be fast)
3. Destroy it with a Pickaxe (It will be like breaking smooth stone)