I come from a country where there are always plenty of open cashiers and no lines. If I saw a queue there I would leave and not go shopping because its either overloaded or some system was down. When I first came to Germany I kept seeing the queue and thinking "oh, I will come back later when it isn't so busy". But I have noticed the reverse occurs too. If there is an idle cashier no one goes to them. They assume, due to the absence of the queue that it isn't open yet or something, so they head straight for the queue.