>> a normal is a direction, so the magnitude doesn't matter,
>> and hence it should be unit length.
>
>  That's a contradictory sentence.
I guess I should be clearer.

A normal could be expressed as two angles, but
is expressed as an x,y,z vector in POV.  Since vectors
have both length and direction properties, but
normals only need a direction property, one popular
aesthetic is to use unit length vectors to represent normals.

In particular about the dark triangles, if the "wrinkled"
normals are all biased in a certain direction, then when
a triangle is near the "edge" then the supplied normals
may point away from the camera, while the actual
true normal of the triangle is still pointing in the camera
direction. You see the front of the triangle from the
camera, but the normal says you are looking at the
rear of the triangle, and shades dark accordingly.

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