I am no expert in this breed in particular. However, this is common in dogs even in the guardian group. This is why I am an advocate for training the dog. Behind the fence the dog will bark and some will growl. It should do those things. These dogs should be aloof. They should be distrustful of strangers. But this does not mean it should attack the strangers once they gain entrance on the property. The dog does not know know what its job is. Until the dog knows it job it will attempt to bluff. If it has been socialized a lot it will accept the stranger because that is what it has been taught to do. IMO without evaluating the dog personally I would imagine that the reason for this behavior is that the dog needs to be trained to do its job. Even in humans we do not wake up and be a great pianist. Some of us are gifted in this area (not me unfortunately). Those that are gifted still need the right guidance and dedication to become what is already in them.

Example: when I was about 4 I began taking piano lessons. My teacher came to the house for lessons. During summers I would spend several weeks at my great grandmother's house. Two of my aunts were taking lessons with the same teacher. So I would switch my day for a lesson and take my lesson there. My younger cousin after my lesson began to try to play what I was playing. I began showing him the song "Traffic Cop". He picked it up fast. I asked him if he wanted to learn to play. He said yes. I went to my great grandmother and told her. He began taking lesson the next week. Soon afterwards he stopped taking lesson with my teacher and began training with someone else. I was learning to read and play music. It wasn't my passion. He began to learn how to hear music and play it. Shortly afterward he was getting paid to play and could play anything. I could only play what I had learned in the book. Today he is an awesome musician and has traveled all over playing. I can only play a couple of song that I remember and one is one that he taught me. He never would have become this if he didn't get the training and put in the time mastering his discipline. The same thing holds true for most dogs. They still need to be taught what to do and when to do it. I can't tell the number of people that tell me their dog will protect them or protect their home because they are naturally protective. If they agree for me to test them to see if it is true they all find out the dog will not. They will back away. With training I can bring out their protective instinct. The bark and growl is enough to scare off most people. But if you want to be sure you should get the dog trained. we should all keep in mind that dogs were domesticated by humans. They were taught to work for us. They look to us for everything including what their job is in our homes. Unless they have been taught to protect by humans or by other dogs they won't know what to do.