What makes the Alien 17 R3 4k IGZO dim automatically?

Say I'm in Dota 2 then I alt tab looking at a white google homepage.  I go back and forth and I see the screen dim gradually like it's autocorrecting.  I check brightness and it's at full for both apps.

There seems to be an ambient light sensor??  sometimes it doesn't dim much at all..

The dim isn't instant but it takes time.  Can someone explain?

It's not overheating because it's cool to the touch.