This is a problem pretty much anyone who has taken a selfie has faced. You look in the mirror and you look good today, your makeup (or just general face) is looking fine and you're feeling yourself. But when you open the camera on your the phone it has other ideas and well, it's not a good selife day. But why is this? Well mainly due to perspective with a bit of angle and lighting thrown in but I'll say more about that below.
And this post does contain quite a lot of selfies so be warned there's a lot of my face in it. 
So it's mainly perspective. And perspective here means your camera sees you differently than you see you, it distorts your face because the closer you get to the camera the bigger you get. Meaning whatever is closest to the camera looks the biggest, shown above with my gigantic hand and tiny head (and I have tiny hands). 
Basically whatever is closisted to the camera (usually your nose) will look big in comparison to what's further away. Making your face look pretty different to what's in the mirror though your bearin doesn't really notice the change other than "ew I do not look like that what's wrong with this". But rest assured it is your camera and not your face.

Now with this gif I used a digital camera and, with my Dad's help, took a picture while changing focal length but keeping my head more-or-less the same size. Basically what I mean is I gradually zoomed in with my lens while taking the camera further away. And as you can see the proportions of my face change massively! At first my eyes look tiny and then my nose and my mouth look out of proportion to my face. If I had a camera with a more varied focal length, so it can zoom in and out more, this would be even more apparent.
Most portrait photographers use a 50-85mm lense when taking photos of people's faces because this is the most flattering, it's closes to what our eyes see when we look at people. But if you go higher than 85mm you'll get people's features looking too small and squashed, whereas, if you go lower than 55mm you'll get people with very large features, like the snap chat filter that makes you look like the dog from UP.
So what focal length is your selfie camera taking a photo at? And surely they should make it a flattering one?
The selfie camera of my phone has a 28mm lens which is great for getting all of your friends in one picture (because it's wide) but not always so great for taking a selfie. 
And on an iPhone it's a 29mm lense which is virtually the same and, again, not the nicest focal length for a closeup of your face.
But phone cameras are limited in the fact they can only have one focal length and the manufacturers want them to be versatile.
For flattering selfies you might just have to use features like beauty face, and although some people consider this cheating I'd just call it fixing the negative bias my phone camera has on my face, because either way it's got it all wrong and I'd rather have the flattering one.
So just to see the difference I've put a selfie from my phone and a (shoddy quality sorry) webcam selfie just you can see the differences for yourselves. And also I don't feel like there are enough photos of my blue hair out in the world.
So cameras are magic and can transform your face, though not always for the best. Next time you're a good mirror day but a bad selfie day, just remember, it's your camera's fault.
And if you experience this problem a lot you will be happy to know that people are creating algorithms and software to fix this selfie distortion and I'm sure at some point in the future they'll put it in phones and then you can look like your real self in your selfies.
I hope this post was enjoyable and maybe you learnt something. Of course there are other things that go into a killer selfie like angle and lighting but I can't put everything in one post. If you do  want posts about angle and lighting (whether in terms of selfies or general photography) than let me know in the comments.
Have a lovely week.
Thanks for reading!