Some textures on Graphics: Fancy / Fabolous ! have some transparent pixels (such as holes in tree leaves), where otherwise on Graphics: Fast, the transparent pixels turn to black. It seems that even while using Graphics: Fancy / Fabolous!, textures such as tree leaves are still LOD'd (made lower resolution) using Graphics: Fast options, so the leaves apear darker on fake chunks than real, vanilla chunks.
Image explanation: There's two birch trees in the middle of the picture.
The tree that is closer to the player, is in a real chunk. It is visible that the fake chunk tree is darker; this is much more visible when you move back a few blocks and the front tree switches to being a fake chunk (both are now darker than birch trees in real chunks), and while being zoomed out.
If this is a bit confusing, please read this again.
If it is still confusing, then ask me here.