Why are some cocoons not boil during harvesting of silk thread?

It must be kept in mind that about 1 per cent of total harvest of cocoons is not boiled. The insects within the cocoon are allowed to mature to form silk moths. These silk moths cut through the cocoons and are allowed to breed for silk worm eggs for future production of silk. The silk from the damaged cocoons is collected and mixed with the filaments of coarse outer portion of the cocoon as well as from the inner portions of cocoons which is left after reeling. This mixture is then spun to obtain low grade silk yarn..