Don't we not fully understand how the body, and the bodies of other beings work? I don't understand how someone can be confident about quantifying phenotype ro

Don't we not fully understand how the body, and the bodies of other beings work? I don't understand how someone can be confident about quantifying phenotype robustness without complete knowledge of the physical properties of organic life. Maybe in the robust category there are small changes to the processes in the organisms that we do not fully understand that build up to break into a threshold of our perception.