Explain me please this behavior of batching static objects. [Video included]

I believe what you're experiencing is a result of the fact that the stats window doesn't update when the game window is not active or unless an editor callback suggests a value has been changed. T$$anonymous$$s is most obvious at 00:53, when you click Cube (1) and the Batches briefly jumps to 10 before changing back to 2 when you $$anonymous$$t Play.

Try repeating the experiment, but t$$anonymous$$s time be sure to click inside the game window after every change you make to see whether the stats values update.

I believe t$$anonymous$$s is caused because Unity rebakes after exiting play mode.
Notice in lower right corner, when you spam play mode, it says "Bake paused in play mode".
If you wait a few second for baking to complete, running play mode again results in 2 or 3 batches.