When played via the Sonos app, it is treated as a music stream. There are sliders in the app to change the music level for surrounds and you have the option of either Ambient or Full music playback (excluding atmos playback)

When played via Apple TV, the audio is treated as TV Audio. Whilst you can also change the level of surrounds separately for TV audio, you cannot switch between different surround modes as you can a music stream.

Consequently, having different surround levels for TV Audio and for Music playback will result in stereo music sounding different between the 2 listening methods, in addition, the Ambient/Full music setting can also change the sound substantially.