In any process that produces physical products, there are always going to be variables and many of the variables will affect the quality of the end product. Products are generally produced in batches and a number of items of each batch are removed and tested for quality process control. If a number of items do not calibrate to the tolerance of the specifications, then the whole batch is discarded.
The quality process control is designed to be an efficient method of testing the quality of a complete batch of items after they have been through various manufacturing processes. It is more efficient to test a number of products from a batch instead of testing every single product individually. Although it does not mean that all of the products will reach the necessary manufacturing standard, it does mean that after testing a percentage of the products from the whole batch, it is likely, enough, that they whole batch of products are made to an acceptable standard.