I agree, I look in the mirror and think "hey i don't look that bad". Then i see pictures and it's not as good.
With a mirror, all you can see is you, so you can normally see good bits and also 'style' yourself to look better in your own opinion. With a photo, you're caught in poses that may not suit you, or, if there are other people in the photo, you are likely to compare yourself and that will alter your view of how good you think you look.