If you had contrary evidence, you might very well consider that, but often you don't.
I don't know if I mentioned it, but before concluding an investigation we will go to the person who is the subject of the complaint and ask them if they wish to give an explanation. Often enough, they do. So not only will I have the evidence for the offence, but I may have the explanation for why it took place. That's very helpful with the public interest consideration.