Should sprites in the same layer but different order incur additional draw calls?

Using Unity Free 4.6.0p1

We are building a 2D game and our enemies are constructed using skeletal sprite animations.

All the enemy body parts are coming from a single sprite sheet.

Clusters of body parts are assigned a distinct order in layer so arms that are closest to the camera don't go behind the torso, for example.

Running the game and viewing the Stats window we're seeing that the enemy is requiring 4 draw calls due to (presumably) having different sprites in different order in layer.

We then did an experiment and put all the sprites in the same order in layer and adjusted their Z-depth to achieve our layering. That resulted in 1 draw call per enemy, which is what we originally expected.

Question is: is this a bug we're encountering? Or is it WAI (working as intended) ?