why does Bruno think of the camp as another city entirely?

When Bruno watched the people in their pajamas on the other side of the fence, he never really thought about who they were, or why they were there. They were a separate entity, liked they "lived in another city entirely." His observations eventually turn to curiosity, and the realization that the soldiers crossed through the fence, but no one on the other side was ever invited over. Thus, the explorer he wanted to be took over, and he began to investigate.