Shrinkage Losses in the raw materials during production

Whenever raw materials are bought and stored, an assumption is usually made that the finished products produced will be of the same weight as the raw materials used during production. Ironically, this does not happen. There are always variances in weight of the end products. The finished products are usually less in weight as compared to the raw materials used. Where are these variances coming from? How can we explain these variances to make professional sense to the procurement on amount to purchase so as to avoid these possible variances?