What's the difference between these stone brick blocks?

Solution 1:

East-facing stairs break into the second type of plain stone brick, due to a glitch.

Some blocks hold a data value, to determine things like the hue of wool or the height of wheat. All stairs use a data value of 0-3 to determine which direction they face:

• 0: South
• 1: North
• 2: West
• 3: East

But stone brick uses this data value to determine type:

• 0: Normal
• 1: Mossy
• 2: Cracked
• Anything else: Not normally obtainable, will default to normal

When you smash a stone brick, it retains its data value, which is why smashing stairs creates different stone bricks. The type of brick corresponds to the direction of the stair it came from. Because there are four cardinal directions and only three types of brick defined, the fourth type of brick just looks like normal brick.

Solution 2:

It might be worth noting that Stone Brick blocks and Double Stone Brick Slab blocks are functionally identical. It's not impossible that, say, the stone brick stairs are incorrectly converting into Double Stone Brick Slab blocks in your inventory, but the Double Stone Brick Slab blocks are being reported by the game as Stone Brick blocks mistakenly.

You might try placing one of each, then destroying each in turn. If both blocks drop identical materials, then something stranger is going on.