(Somewhat related to MC-46051)

Minecraft's shading system darkens block faces by the direction they're facing: a block's top face is the lightest, followed by the north and south faces, then the east and west faces; the bottom face is the darkest. The same goes for entities.
In item frames, however, an item's "top" side is the lightest (no matter if the item is rotated), then the front face (regardless of the orientation of the item frame itself) then the "side" faces, and the "bottom" side is darkest.