Adding ONE shadow caster and ONE shadow receiver to my scene DOUBLES my draw calls. What?

I'm looking at a scene which, with no realtime shadows at all, renders in 59 draw calls. We've basically got a character standing on a ground plane, and a bunch of environment props scattered around him.

If I enable "cast shadows" on the character (and JUST the character), and "receive shadows" on the ground (and JUST the ground), my draw calls for the scene increase to 101.

The character is literally the ONLY shadow caster in the scene, and the ground is literally the ONLY shadow receiver. If I factor out draw calls for the GUI, it looks for all the world like I'm getting an extra draw call for every object in the scene -- even objects that neither cast nor receive shadows -- if any shadow is drawn anywhere at all.

Shouldn't I only be getting extra draw calls from the character's shadow caster? Where is the rest of this coming from? O_o