The Caffe models are only trained on the 1000 category subset of ImageNet used for the competition: http://image-net.org/challenges/LSVRC/2014/browse-synsets

There are no sofa's in this list, the closest thing I can find is a "studio couch, day bed": http://imagenet.stanford.edu/synset?wnid=n04344873

Right. The "studio couches" are couches.