Band 5 - The candidate gives short answers
Band 6 - The candidate is able to speak at length and is reasonably fluent
Band 7 - The candidate speaks fluently without much effort
This means that if your answers are short you are only worth a 5, even though you are fluent and confident.
If you speak fluently and at length, you are still restricted to a 6 if you have to make a visible effort to speak.