4K Apple TV dark in Dolby Vision?

I have a question I have a Apple TV newest generation. I have a Sony TV 4K HDR model.
When I turn on 4K Dolby Vision on my Apple TV the menu area gets darker. This is the highest video quality on my Apple TV. When I turn on 4K SDR it is brighter. Why is this. Does the Apple TV automatically run Dolby Vision???

I love the show been listing since the beginning when I do my walk.