Another hypo: it is a confidence issue because you know you are an expert of what you speak about and the audience is interested and how good a presenter you does not matter so much. So you are full of confidence. You know you are giving them a good product, you know they want to buy this product, maybe the packaging is not so good but that is okay.

For the other situations, you don't know the product is good and you don't know they want to buy.