I've gone 1/54 before on T1 > T2, granted that's not an average just the worst I've done, by the same token I've also gone 1/1, I'm just pointing out his example is possible.
As to the OP I'd love to have multiple VNMs as well but I can see why it is 1 only every 10-15 min, instead of killing 12 different NMs X times you get 5 the number of different monsters is less but the difficulty is artificially increased when competition is factored making them comparable.