One reason is that some of the questions involve public choices, not just science. Another reason is that science isn't tidy.
Scientists often disagree with one another.
It turns out we were wrong. Large woody debris like logs provides important habitat for salmon. Almost certainly some things we conclude now will prove to be wrong years from now when more information is available. But in science the only way to avoid being wrong is to do nothing, and that's certainly wrong. Martin uses a medical analogy.