It's probably not necessary at this point, but I'll add on to the list of variances that Jessica mentioned regarding photos. In the film days, the type of film used made a huge difference in terms of how colors were captured, the amount of "noise" or graininess, even the overall sharpness of the image. Well in digital cameras of today, even the ones on your phone, it's even more complex. The sensor that captures the image can cause many of the same variances that film did. However, further complicating it is the translation of what the sensor captures into the final image file. Each camera, even the one in your cell phone, has a processor that applies certain algorithms to the photo to adjust things like color saturation, hue, sharpness, white balance and other factors. Samsung phones for instance are infamous for raising color saturation to unnatural levels. The iPhones tend to over-compensate in sharpness leading to unnatural edges. Canon digital cameras have always had a reputation for some of the best image processing chips in the industry but even still people sometimes complain about different aspects. Now add to all this, the fact that on most cameras, even your iPhone or Galaxy Sx, you can adjust every one of these parameters, the end result of a photograph is rarely a truly accurate representation of reality.
When it comes to mirrors, lighting is every bit as important in what you see as it is for the camera. Just the type of light bulbs being used makes a difference. A lot of people don't even realize that different bulbs have different color temperatures that can affect how you see things. A cool white bulb (high color temperature, more pure white) tends to wash out color on the face. In fact these types of bulbs are not recommended for make-up mirrors because they cause you to over do it on blush and bronzer. Soft white bulbs (low color temperature, more yellowish light) produce a more balanced color on your face but if they're too soft, can actually accentuate the color of your face. And of course natural sunlight is another animal all together but is ultimately the most accurate. Of course where the bulbs are placed makes a different too in terms of shadows cast which even if very subtle, will also cause differences in perception.
So what I'm saying is, you can't really expect either a mirror or a camera to give you an idea perspective of how others will perceive you. Each situation will be different and you can go insane trying to level that out (trust me, as a former professional photog myself, we sometimes do). So just use both as a tool to give you a good idea of what others see but keep in mind they'll always see something just a little differently than you do.
