If they asked me tomorrow what date I first started conversing with my wife -- I'd forget. But there again, they don't care. It's already been processed. They have already assessed the bona fides of your file. Your relationship was already assessed. Why would they waste time at a landing to ask that stuff??
Don't read too much into this. It will only be harmful to you. You've been approved. They just need the interview to give you the COPR and to validate your basic info is correct. That's all -- no bait and switch, or smoke and mirrors.