It's ALL about the lighting and the shape of the mirror.
When the light source is behind or to the side of the person, they tend to look less attractive. Because the light can not high lite your facial features as if it was in front of you.
When taking a picture make sure the light source is in front of you. (The light source does NOT include flash on a camera but a light bulb or the sun, any main source of light).
Also, when you use a wide shaped mirror than your face tends to look wider, if you buy a thin shaped mirror than your face tends to look thinner. Mirrors proportion your face differently depending on shape.
Also, it could depend on the type of lighting. Most people prefer natural lighting as opposed to fluorescent because it can distort skin tone and make it look less "natural".