Porcupines' quills, or spines, take on various forms, depending on the species, but all are modified hairs coated with thick plates of keratin, and they are embedded in the skin musculature. Old World porcupines (Hystricidae) have quills embedded in clusters, whereas in New World porcupines (Erethizontidae), single quills are interspersed with bristles, underfur, and hair.
Quills are released by contact with them, or they may drop out when the porcupine shakes its body, but cannot be projected at attackers, contrary to popular belief. New quills grow to replace lost ones.

Information from --> http://en.wikipedia.org/wiki/Porcupine

( they may drop out when the porcupine shakes its body )

This may be why, people think the quills, can be projected.