Breaking non-oak doors yields an oak door item

I crafted and misplaced the wrong door type ((Spruce when I wanted Dark oak)) so when I went to remove the door it yielded an Oak Door ((or just "Wooden Door")). I crafted several spares of those three types ((Birch, Jungle and Acacia are untested, assumed to have a similar result)), and placed two of each all in a row. I broke one of each pair from the bottom block using an axe, and the others from the top block using an axe; all six tests yielded an Oak/Wooden Door.

EDIT: I have now tested it with Birch and Acacia Doors, with identical results.