Why two spheres give two draw calls while two cubes don't?

I read that all gameobjects sharing same material will give one draw call all together.But I am having a hard time understanding all this.I tried texture atlasing to reduce draw calls but its not working. It only seems to work with planes and not with other 3d models which I export from maya. And also scaling the plane again also increases the draw calls.
Tried the same with primitives in unity. Cubes with same material give only one draw call while spheres with same material don’t. Would really help me if someone can clear all my these doubts.

I read that all gameobjects sharing same material will give one draw call all together.

That’s not true, or rather it’s only true in specific circumstances. Unity - Manual: Draw call batching Spheres have too many vertices.