Tusq saddles are made from a high-quality polymer, formed by high levels of heat and pressure. Every single Tusq saddle will be exactly the same, which provides benefits for guitar manufacturers, particularly those producing a large number of instruments.
Conversely, bone is an organic material and, just like the tonewoods on your guitar, each bone saddle will be subtly different, although with quality bone saddles, that variance will be lessened from piece to piece. Inexpensive bone saddles will see a greater variance between piece to piece because the selection of bone and quality control processes are less demanding.