When Cobblestone Slabs are stacked, the texture is lighter than a normal Cobblestone Slab in the upper position.  If Double Cobblestone Slab is broken and a normal Cobblestone Slab is places in the upper position, it will keep this lighter texture.

Also, noticing if a Cobblestone is removed and replaced, it takes on this lighter texture.  If a block is place atop the darker Cobblestone Slabs, it will change the lower one from darker to lighter.