I know what you mean

Have you ever told a story about something and the person who is listening to you responds by saying "I know what you mean?" I think that people don't really mean what they say sometimes because they might just say it for no reason. On the other hand, when somebody is talking to me about a very important issue that I feel so strongly about, I will repeat these words knowing that I actually feel a certain way. Today, I was talking to my little cousin about motivational speakers and how their insights are very important. They know what they are talking about and help us see the world in a very broad view. Well, anyways, she turns to me and says "I know what you mean." So, I keep going thinking that she understands what I am talking about. At the end of my story, she nods her head and says "I have never listened to a motivational speaker." I was really mad at that moment because I thought she understood me and knew what I was talking about. It is a very vacuous reason to be upset about but why would you say something that you don't really mean. I am glad she at least paid attention to me and nodded once in a while. At least she mean that? Or so I think?