Lower quartz slab jumps up a block space when placed on snow/frost under another lower quartz slab.

What I expected to happen was...:
The quartz slab should stay in the lower block space as a lower slab.

What actually happened was...:
The quartz slab jumps up a block space when placed under another lower quartz slab.

Steps to Reproduce:
1. Place a 1.5 block tall 1x1 pillar of quartz slabs down on showy ground in creative
2. Place snow around the pillar
3. Place a slab extending out from the top slab (they should both be lower slabs)
4. Attempt to place a block under the protruding quartz slab

Pictures attached
2013-03-09_02.03.35.png - Before placing the quartz slab on top of the snow under the protruding quartz slab. Aim at the spot in the picture.

2013-03-09_02.04.06.png - The Result. This will only happen when aiming at the snow underneath. If aiming at the lower half of the double quartz slab block, extra slab does not jump up.