After placing slabs on the lower level, if i place something on top it floats as if the slab was a block

I was building a roof on top of a tree, I placed down lots of wooden spruce slabs, then when I placed any item on top, i.e. planks, fences, chests etc. they floated 1/2 a block, as if the slab was a full block.

What I expected to happen was...
When I place things on top of slabs they go directly onto the slabs without floating.

What actually happened was...
When I place things down they float 1/2 a block

Steps to Reproduce:
1. Place down slabs on the bottom half
2. Place another type of block, on top of it,
3. Watch in amazement as it floats