This is probably a big reason why you see tons of articles about the next big treatment for X or Y coming out, and then the treatment/device is never heard about again. Reproducibility is and always has been a huge issue in science, and animal testing is no different. Although many different models exist to try to homogenize the process, there are so many steps along the way that contaminants can enter the process(breeding conditions, feeding the animals, genetic mutations, where the animals are sourced from). All of these things can cause differences in results for the same study when the species of animals are the same, not even considering when you jump from one species to another. However, there is no real substitute yet to replace this method of animal testing, so for now this is probably our best course of action.