Why? Well, I like to convey to my auditee that I like them, that I trust them. It builds a rapport, and most of the time it is true. I usually do not have a reason to distrust a person’s word. What I distrust is their memory. Because they may think that they did something, but never really got around to it. Or they recall that it happened “this way” but it occurred “that way”. More often than not, the issue is usually the timing: “Oh we just purchased that a couple of weeks ago, there is no way that it could be expired already.” And then when you check the purchase records, the item was actually purchased 12 months ago, and therefore extremely plausible that it could have expired 3 months ago.
You can trust what “they” say, just make sure that just as with every other piece of information, ensure that you verify, verify and even possible verify again. Then be open with your auditee, with what you found, that you found a discrepancy. Allow them the opportunity to review and discuss it with you. Instead of having them get defensive with an accusation, and possibly shut down the rest of the audit. Get them to become champions in their own change and improvement.
Remember the definition of evidence: Information that has been verified and that is relevant to the scope and criteria if the audit.