Slab's shadows are too dark

Slab's shadows are too dark.

What I expected to happen was...:
I though the wall won't have 2 differents colors

What actually happened was...:
The slab has a darker shadow.

Steps to Reproduce:
1. Make an angle with 2 walls (2x3).
2. Create a roof with slabs on high position.
3. Put blocks on top of the slabs.