Are you sure you didn't opt in?

Yes, really. I’m sure I didn’t opt-in.
I get a lot of spam. I get a lot of spam to addresses that aren’t used to sign up for mail. But it seems inevitable that when I bring up examples of receiving spam I inevitably get asked, “Are you sure you didn’t opt-in?”
On one level I can understand the question when I send in a complaint to an abuse desk and they’re dealing with a customer who swears all their mail is opt-in. It makes sense when an ESP is working to identify what may have happened so they can correct their customers’ behaviour.
But when it’s a client who has hired me to investigate their email delivery problems and I provide examples of spam sent to me? Why, WHY would I lie to you? Why would I claim I’m getting spam if I wasn’t? What use is that? How does me forgetting I subscribed actually help fix your delivery?
And even if I did forget, shouldn’t that be a sign that maybe there is some issue with your mail program that people sign up and forget?
I am not sure what causes clients to think I would tell them they’re spamming me when they’re really not. I certainly do tell clients when I opt-in and enjoy their mail while offering advice on how to improve their marketing program. I’m not sure what’s going through their heads when I say, “Oh, you (or your affiliate) is sending me a lot of spam,” that prompts them to ask, “Are you sure you didn’t opt-in?”