I thought this was superflat!

When I ventured out in a superflat world (preset was overworld), I saw the unthinkable. A Forest Hills biome in a straight line!

What I expected to happen was...:
Everything would be flat except for generated structures.

What actually happened was...:
In a line was a Forest Hills biome!

Steps to Reproduce:
1. Create superflat world. Preset MUST be overworld.
2. Once you spawn, venture out in any direction
3. You will find, eventually, a Forest Hills biome!