Double half slabs are all stone

Whenever a 2 half slabs are placed on each other, they turn into a double stone half slab. (Doesn't happen on wood slabs)
How to re-create:
1) Take 2 slabs that are not wood.
2) Place them vertically next to each other.
3) Once both slabs have been placed, the double slab turns into a double stone slab.
Thank you for your time.