It's the same phenomenon as you think you look fine in the mirror but then you see a photo of yourself on the same occasion and you think you look awful.
Most people when looking in the mirror or posing for a photograph, consciously or unconsciously put themselves into a flattering position, for example:
- tilting head to show 'best side' or disguise double chin
- Smoothing hair or pulling it forward over shoulders to look thicker
- standing up straight
- pulling shoulders back
- holding tummy in
- smoothing clothes, making sure everything is tucked in or not tucked in if it shouldn't be
- straightening legs to look longer, doing the 'one leg forward' pose to show hips at a flattering angle
- pouting/sucking cheeks in to show cheekbones
- checking any make-up, refreshing lipstick, blotting shine
- moving if lighting is unflattering
Candid shots have none of that and it's very much a matter of luck whether you are 'caught' in a flattering pose.
Particularly if it's on a night out, as the night goes on you might become more dishevelled; if booze is involved you may end up with red cheeks, drooping eyelids and so on due to alcohol relaxing your muscles.
In the era of digital photos, if you take a selfie and you don't look good, you'll delete it, forget it and take another - so your self-captured portrait history always shows you at your best.
Friends taking photos or sharing photos taken by others will usually only care or notice whether they themselves look good, hence the group photo on Suzie's Instagram where Suzie looks like she's stepped off the catwalk and everyone else looks like a whale on a bad day.
The important thing to remember is that just as the camera can 'lie' and make anyone look great at the right angle and in the right light, it can also make anyone look rubbish if those things are wrong. Think of the 'candid' celebrity poses you see where the Daily Mail etc. posts some gorgeous actress captured first thing in the morning in jogging bottoms with her hair screwed back in a ponytail looking 20 years older than she normally does.
Your daily self, what others see in real life, lies healthily between the two.