I don't think it can be changed, it is the way the lighting algorithm is made. The amount each vertex is lit is based on the difference between the angle of the light, and the angle of the surface normal of the triangle (which represents the direction the triangle is facing). If there is too much of a difference, there will be no light at all (because the triangle is backfacing). If the triangle is facing directly at the light, it will be fully lit. Any where in between and you will get different intensities of light.
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