Slabs and some stairs are different colors than the block

While I was building, I noticed that the slabs and stairs were a different shade than the block of the same material. I made sure it wasn't just a shadow casting on the slabs or a lighting issue. For example the spruce plank block was a lighter shade than the spruce slab. The slabs are much darker than the blocks that many of the builds look funny. This happens with almost all of the slabs, but not so much the stairs. I have provided an example of the spruce plank, cobblestone, oak plank, and acacia plank blocks with the corresponding slabs on top of each block.