Why the instance id picked with "Scene.pick" from primitive is not exact?

The official website tutorial has the following content about " Custom Geometry & Appearances".

Instances are independently accessible after they are combined. Assign an id to an instance and use it to determine if the instance is picked with Scene.pick.

But I find that the pick.id is not the id of the geometry I clicked but the id of other geometries.
For example, I clicked the left geometry but i obtained id of the right geometry in red box.
Why does this happen and how can I get the exact id?
Any feedback would be appreciated!