Not being one of those primary researchers I cannot speak for them, but it does not follow that researchers have to agree for at least a couple of reasons. First, the research might not be complete. Second, even if a certain stage, say structure of trouts' eyes, is complete, there are other factors. Perception is not merely a matter of the function of the eyes, but also of the brain. I suspect that's why it was taught in my intro psychology classes. That is one of the reasons why certain types of observers, such as snipers, are taught observation so rigorously. Perception involves both the physiology and the psychology of the perceiver.

I have omitted mentioning a third reason. Some of the researchers may be incompetent or have some reason to obfuscate the results. But then that is my own, potentially flawed, perception.

Regards,
Ed