Made a superflat world with 1 Bedrock, 30 Stone, 5 Dirt, 1 Grass. Turned on cheats and bonus chest. The world spawned with 2, 1 Chunk wide strips of normal and the rest superflat. The superflat grass was affected by the biomes.

EDIT: At the end of the two strips they are connected forming a hollow "rectangle" of normal land. Upon recreation under the same circumstances it was normal. Weird?