Overlapping objects increase drawcalls in 2DToolkit

I have 3 images overlapped as a background. They overlap with different "Order in layer" value. They all make 1 drawcall. I got 1 drawcall more for the camera, it makes 2 drawcalls in total.

I created a prefab with an image from the same collection as the backgrounds. It has a different "Order in layer" to overlap the background. I instantiate the prefab. I get a total of 4 drawcalls in the scene. I can instantiate how many prefabs I want and the drawcalls are always 4. Why is this? The sprites are from the same collection, it should not create more drawcalls (and why it takes 2 more drawcalls to render 1 prefab if it got only 1 sprite!)?