Well, you see... When one has a Hydra, the problem becomes less of a rule of additives, and more of a rule of multiples. Say you have one Goblin, and I have one Hydra. Now, the interchangeability rate is modified greatly due to the existing factors of three Harpies per one Goblin on hand. This in turn is further multiplied by the law of in-congruence due to the fact that a Hydra has a base value of seven Harpies, so therefore an equal but different approach is realized by multiplying a Hydra by two Oracles. Now the real issue becomes the added variables, that is the Harpies and the Oracles. Each Harpy must then be divided by precisely two Oracles to reach a desired product ratio of six Oracles per one Harpy. So you see, the multipliers become so grossly exaggerated that, given time, even the Oracles will be fed up with this equation (and trust me, it's hard to get an Oracle to show any emotion towards a subject) and decide in the end to consign you to the depths of time, where you will be eaten by Sir Isaac Newton in a parallel reality in which the apple fell and he caught it perfectly between his jaws. So screw you, gravity.