Sonos ran extensive tests and found that was the optimum level?

Not for music, that’s for sure 

Maybe as a TV audio device it is tuned optimally for movie rather than music.

When I had a Sonos Amp setup for 2.1, regardless of if I used my Sub Mini or 3rd party sub, there was a big difference between music playback and movie/tv playback. With the sub set the way I wanted for music it was overwhelming for movie/tv. Set at an appropriately for movie/tv is was too low for music.

I just put it down to Sonos choices on turning/audio processing because swapping the Sonos Amp for an AVR running 2.1 meant I setup the speakers and sub once and it was right for whatever I played back.