Bedrock at Y0 has wood log in it at x0 y0 z0.

Decided to plant a tardis interior underground at the origin. Was surprised to see a log in the bedrock at x0 y0 z0. Tried with a newly created world and it was there too. Doesn't seem to be there in maps from 14w27. I used "Log at 0 0 0" as the seed the second time.