This is actually a great question. I believe that it has to do with the shorter signal path (UPnP App-> Player -> Internal SSD) vs Roon which requires a lot of back-and-forth traffic between core, player, etc. When you condition for good power, low noise, and jitter, issues are reduced a lot, but not 100%. Of course, this is just speculation based on a purely pragmatic approach. Also consider that while there is a difference that most will be able to tell easily, it is between great and greater, no bad choices IMO.