As I understand it, there’s no absolute “standard” volume/signal strength they are obliged to meet. It’s the same problem with volume normalisation across music stations. 

And Sonos plays the volume it is ‘handed’, it doesn’t do any compression to make each channel the same relative volume. 

You’re equally unlikely to get a satisfactory answer from your cable company as to why they send the channels out with different volumes. They’ll likely tell you they just pass on what they’re handed by each channel. 
And I don’t think each channel checks with others to see what ‘volume’ level they’re sending out, as @nik9669a points out. 

Thanks for your helpful reply Bruce

Not really sure it’s all that helpful, but you’re certainly welcome.