Why were some cars built with 10 bolt or 12 bolt rear ends?

Thanks

 

Probly a option that you can get when you special order you car from the factory. Im just guessing here. hope it helps 80z/28

 

12 bolts were used on the more powerful or hevier cars, and the 10 bolt was intended for the less powerful. During the late 70s and 80s there were no powerful cars, so GM stopped building the 12 bolt. When the cars got more powerful, in the 90s, GM had nothing but the 10 bolt to go with.