What do you mean when you state 'unders & overs' in your Terms & conditions?

Due to the nature of printing and the materials used, it is not always possible to judge the exact amount of products that will be produced & printed specifically for your order. Also we need to take into account spoilages and machine stoppages. Therefore, our terms and conditions state that your order is subject to unders or over of up to 10% meaning you could get 10% more or less.