1 The Eye is a more powerful camera
The Human Eye is Equivalent to a 576 megapixel camera. Because Cameras usually don't have the power of a human eye they don't show people's real looks.
2 Lighting, shadows & depth of field
There are many factors that change a person's looks in photos and videos such as Lighting, shadows & depth of field, See also Why you look bad in pictures.
3 Properties of the Monitor's Screen
If you are using a webcam then your might change depending on the different properties of the monitor. Brightness, contrast, gamma, hue and saturation can affect how you look in a webcam.
4 The Camera angle can change your looks
People can look different in pictures based on the Camera's angle. Playing around with angles can result in great change sin appearance, See why some people hate to appear on pictures.
5 Video Resolution is usually lower
On a typical mobile Phone video resolution might by lower than picture's resolution. This can make pictures in videos seem more ugly when compared to normal pictures.
6 You see yourself from unfamiliar angles
We feel comfortable the most when we see ourselves from certain angles. Because videos change those angles we tend to find ourselves less attractive, See why sunglasses make us more attractive.
7 Your picture is reversed
Mirrors reverse our pictures. When you see your picture in a video it's the unreserved form. Because this picture might not look familiar it might seem ugly, See why we like familiarity.