Unity considers single mesh as 3 or 4 shadow casters?

I'm trying to understand why a single mesh increases shadow casters statistics by 3 or 4.

1. Create empty scene. Create a ground using unity 3d cube (cast shadows off). Run game and stats show that shadow casters = 0.
2. Add a cube on top of the ground (cast shadows on). Run game and stats show that shadow casters = 3.
3. Add another cube. Run game and stats show that shadow casters = 6.
4. Add another cube. Run game and stats show that shadow casters = 9.
5. Select ground, turn on cast shadows. Run game and stats show that shadow casters = 13.

I don't know exactly what unity considers a shadow caster and I can't seem to figure it out from the docs. I assumed a shadow caster was an object that casts shadows. But according to the rendering statistics, that can't be correct since there are 13 shadow casters in the scene but only 4 objects that cast shadows.

The docs also don't mention shadow casters in the rendering statistics so it doesn't say either.
https://docs.unity3d.com/2017.3/Documentation/Manual/RenderingStatistics.html

Is something not working right or am I just not correctly understanding what a shadow caster is?

 

The main directional light uses cascaded shadow maps. That means it renders the shadow map for the directional light multiple times at different ranges. By default Unity uses 4 cascades, so a single shadow casting object may be rendered between 1 and 4 times depending on how the cascades overlap.

Check out the frame debugger window.

 

Thank you so much for the very quick and very helpful reply! After reading your post, I looked in the docs to learn all about shadow cascades which was exactly what I needed. You're AWESOME! Thanks