language Differences

In class we discussed language styles and differences for a portion of the day.  This made me think about when I was a freshman in high school.  My girlfriend at that time was from germany, she spoke fluent english and had lived in America since she was 9 years old.  Though she spoke english to me, her parents still spoke "deutsch" aka German to her.  It was quite interesting to go over to her house and hear them speak it to each other.  The only thing weird was, i was once talking with her and her parents, and all the sudden her parents would turn to her and begin speaking German.  This got me quite paranoid.  Later I asked her what they were saying, and she simply said that they misunderstood what I was saying to them.