Being Able to Create Stone Slabs with Andesite,Granite and Diorite Using Mattis System

I have been playing around with andersite, dioride and
Grandite and saw that the Mattis allow the creation of stone slabs without stone I am wondering as it is a feature or a bug as stone slabs cannot be crafted with These three types of stone
*Note I did not have any other types of blocks like stone in my inventory.