There were sporadic chunks inside of a swamp biome that were ocean biomes (said so on F3, wasn't just based on observations)

I expected a normal swamp and got the one that was littered with ocean chunks. The chunks were randomly placed

1. Loaded up a new world
2. /tp ~15000 blocks x
3. /tp back to 0 x 0 z and observe

seed: 7520406132772873981

Optifine was installed, but not when the world was first generated.