Most likely he was trained to respond to that when he was younger. Many people use sounds for commands rather than words. From the sounds of it someone has trained the dog pretty well, but nobody else knows the commands he is used to so the dog is confused because he is getting mixed signals in his mind and doesn't really know what the words coming out of your mouth mean. Have you been consistent in treating and rewarding when he responds correctly? That is the only way he will put 2 and 2 together and understand. You have to remember he is just a dog..they aren't born knowing right from wrong and they certainly don't know the rules of the house when they get there and are quite confused.