Why the overdraw mode behave like this with geometry rendering?

I am trying to understand how Unity’s overdraw mode works and what it represents. I created 2 3D spheres using the default material, which is rendered in the Geometry queue front to back. I placed one sphere behind the other.

So, I expected to see the 2 spheres have the same color in overdraw mode. However, the overlapping region turned out different color as if the closest sphere was rendered after the far away sphere. Frame debugger shows that this was not the case.

What am I missing here? Isn’t the overdraw mode supposed to show that these pixels were drawn once?

Overdraw: render objects as
transparent “silhouettes”. The
transparent colors accumulate, making
it easy to spot places where one
object is drawn over another.

Edit: So I just checked it myself and I got the same results.

Seconded. I’m seeing the same thing on my end too and it makes no sense. Could it be a bug?