Surround sound function on Apple TV

What exactly does surround sound on/off do to audio? I looked at my receiver and on both settings the receiver reported input of 5.1 (3/2/0.1).
I notice that dialogue is louder with surround sound off. Or is that my imagination?

Love the Apple TV.
Doug

Off means convert to two channel before sending to tvOS

Maybe something is upmixing on your setup afterwards