Strange behavior of three ligths

Hi all!
I'm a newbie in Unity. I learn it and try to make simple scene that comtains a plate, box, sphere and three spot light sources.
One and two spot ligths work fine and predictable:


But when I turn on any third source it behaves like I turned on only the third one but also some additional light that illuminates the whole scene:

It is a bug or feature? What have I missed?

 

try set render mode of all lights to important (in editor)

 

Yes, it works. And forces me to RTFM
Thank you.