Sitting shows you are not rushed
I am an american medical student and at my school, we are taught to sit in all patient encounters, even if you have to pull up a chair from outside the room (this of course is referring to actual patient encounters, not this exam!). In this way, we demonstrate that we're comfortable, we have as much time as they need to explain why they came in. If you stand, it sends a signal that you may only be there for a minute or two and you may be in a hurry (which we may be, but they can't know that!).