Variety is achieved by not using parallel intervals. To repeat a perfect consonance would be like attempting to repeat perfection. Zarlino exercises himself quite a bit in his attempt to give an account why consecutive imperfect intervals of the same size should be avoided. However, from his examples it seems clear that, in most cases, what is involved is a cross-relation. That being a B natural and a Bb in two diff.
Voices close together.
This rule may have to be ignored as voices get added, to keep the horizontal motion singable. As voices get added, Chapter 30 becomes less important because you can fill in the voices to lessen the harsh blow a tritone might cause. This goes for all intervals. And the singable intervals are those that are close together. This avoids parallels and softens the texture.