First, let me say I really do find your question interesting. I gave it a few minutes of thought and have some ideas. Obviously, they may be wrong--just my opinions =)
1. When you're looking in a mirror, YOU have more control over what you see. You may only see your face, you may have different lighting conditions, heck, the mirror might be full of steam from a shower/bath!! lol! (That always makes ME look better--when I see myself in a fully steamed mirror b/c I HATE looking at myself even though I've been told I'm not that hard on the eyes)....
2. In photography, usually someone ELSE is taking the pic. Often times, it is NOT a professional photographer. Also, the optics are different on film or pic versus a reflection.
3. I truly don't believe the majority of people who don't "like" their looks would say they look better in a mirror as opposed to a picture.
4. Pictures may be either unexpected, poorly posed, poorly lit, showing bad angles (poorly composed), etc. It's said that photographs add weight as well.
However, while I believe all of those reasons may touch on possible answers, my biggest belief is we have more control over how we look when looking in a mirror.
Still, I love your question!!