darker shading (backwards?) on copied parts with correct (?) normals

Sofa example.blend (504 KB)

So I’ve seen this happen before, but now I want to know why/how to fix it.
These two sections are from the same mesh, just copied and moved next to it.
You’ll see one looks darker than the other. If you flip the camera to the other side at the same angle the shading switches, then the lighter one is darker etc. It also happened when I tried to do a mirror transform, after flipping the normals back, it still would look like this. It’s like the shading is backwards, yet the normals “seem” fine.

Attachments


The left side of the original object is affecting its shading. Separate it, and you’ll see the difference. Alternatively, if you are going to rely on sharp edges, turn auto-smoothing on (though you’ll have to mark some more edges).

Has nothing to do with those, but thanks.
Also I’m referring to the overall darker or lighter color as opposed to any smaller shading anomalies (which blender is famous for), which perhaps was not clear with my use of term “shading”.