Cognitive dissonance is widespread. An old roommate once informed me she had given up drinking, because she switched from vodka crans to wine. It's similar to vegetarians who sometimes eat chicken and people who "don't watch TV" because all the shows they watch are on Hulu.
I'd suspect it's less to do with cognitive dissonance, and more intentional lying to meet social norms or out of fear of future problems.
Anything controversial, highly personal, or with stigmas attached will get biased results from self-reporting. Alcohol, drugs, sexual history, religious views, political views, racial prejudices, etc will almost always be biased towards what the person thinks is the right answer.
