/execute from half slab rounds Y down not up

"/setblock ~ ~2 ~ dirt" places above your head, even when on a halfslab.

"/execute @p ~ ~ ~ /setblock ~ ~2 ~" also places above your head, except not when on a half slab. It places in your head.

This also can be seen if you:
/summon Fireball ~ ~2 ~

{direction:[]}

/execute @p ~ ~ ~ /summon Fireball ~ ~2 ~

{direction:[]}

Video of setblock for evidence / unmistakable ability to recreate:
http://youtu.be/ekbPS_XoCV4