I don't think it's intentional -- it probably just emerges from the training process. I'm guessing they do something like load up a batch of images and once N

I don't think it's intentional -- it probably just emerges from the training process. I'm guessing they do something like load up a batch of images and once N people agree on one, record the answer and remove it from the rotation. You end up left with the ambiguous images where people couldn't agree.