Grass Blocks Underground in Caves

I found multiple grass blocks in caves underground where they shouldn't be.

What I expected to happen was...:
Find dirt blocks underground in caves.

What actually happened was...:
I found grass blocks in a cave.

Steps to Reproduce:
1.Generate Survival World(This might work in creative too)
2. Find cave
3. Find grass block in cave

Note: The world I found these in was generated in 1.6, but i found them while playing in 1.7.2.