Two different input methods. One is coming through the TV input (usually HDMI) and treated as regular TV input (Sonos doesn’t ‘know’ what is coming in, just that it is). The Sonos will play whatever it is handed, without converting. So if the TV is handing it stereo, it will play stereo, mostly through the two ‘front’ speakers in the soundbar. 

However, when you’re streaming via the Sonos app itself, the input is coming from the servers owned by Spotify directly to the Sonos, without being filtered by the TV. Since Sonos ‘knows’ what kind of stream it is, you can easily set the surrounds to ‘full’ rather than ‘ambient’, and get full stereo out of both the sound bar and the surround speakers. 

Just as a note, I’ve done a bunch of simplifying there, but the basics are correct. Two different input methods to the Sonos. And, two different ‘players’ as well. 

Got it thanks Bruce for your time comments. Sonos is great for sound and when it works it works well but it comes with its own intricacies. As it just happened I switched Spotify over from my tv to Spotify on the Sonos App and lo and behold I am now getting a stronger fuller sound through my rear SL ones 

As I said, two different inputs. So the signal is treated differently. Glad you’ve found your preferred method.