5.1 Audio from Stereo Sources

Deep tech question.  I have an XG1v4 DVR.  When I play content from third party apps like YouTube and Spotify, which only do stereo, the XG1v4 output is 5.1.  When set to Expert mode, I get 5.1 PCM.  When set to Auto Detect, the output is DD+ 5.1. 

So, what is Xfinity doing that takes a 2.0 source and sends it as 5.1?  Is there someone in tech support there who might be able to explain this?