Trees Growing through dirt

Trees, oak is what I tested, will grow through dirt blocks when set as a height limiter. I tested it in single player creative, but it happened to me in survival on a bukkit server without mods as well.
Tested 3 types of blocks, dirt, cobble and oak planks, same height, same number of trees. Cobble and oak planks seem to work properly while 2 trees grew through the dirt block line.