19/20 times act bosses will drop their corresponding statue.
1/20 times they will drop a statue that is +1 or -1 their corresponding act (for example andariel can drop act 1 or act 2, duriel can drop act 1, act 2 and act 3 and so on). Diablo can drop act 3, act 4 and act 5 statues.
The 1/20 drop chance can roll multiple times. People have posted getting 3 statues at once before.
I’ve had Diablo drop two bk statues multiple times now. Would be cool to see a worusk and bk drop together