mesh normal

Because the normal is pointing away from the light, so the face isn't lit. Remember that lighting is done as L dot N (Light vector dot Normal vector), so if the normal is facing away from the light, you'll get a darker triangle.

The best way to fix it would be to fix the normals so they're not backwards. Also, not culling back-facing triangles could cause some performance issues and is best avoided for this purpose.

You might want to try googling/searching for "two sided lighting" - I know Nvidia put out some samples a few years back (it was one of the cool new things that vertex shaders made easier). That should allow you to have a single triangle that can be lit from both sides - although typically its used so you can have different textures/colours without doubling the geometry.

hth
Jack