Due diligence is the checking, what you are looking at, is what you think it is.
It is actually quite impossible to be one hundred percent certain about anything.
However this being the case is no excuse for not trying at all. Which is why the term is phrased this way to indicate that Due, meaning the necessary amount of checking in a particular act of diligence, to be comfortably certain that what you are looking at is what you think it is. The terms total or absolute cannot be used, without creating a misrepresentation of the facts.