When looking at the Creative inventory (Decoration blocks tab) you'll notice how the chest, furnace and ender chest all show the front side, pointing to the lower right corner.
However, all Skull blocks (#0397/0 to /5) are faced to the upper left corner, which is 180 degrees around. This looks like a bug to me.
Another way to reproduce: /give @p skull 1 5
Meh, this turns out to be a duplicate of MC-109831.
I was searching for 'skulls' not heads.