I could be wrong on this but I believe .NET Standard is 'portable' but not PCL. So it does not adhere to the old PCL standard, but is portable across platforms hence visual studio give you the tag.
That's a guess on my part, but I think I'm on the right track.
As a fun side, I had zero success using VS 2015 to create a secondary Xenko/portable library to sit alongside my game library. Had to hack around it by copying the existing game project to a new location, opening it, gutting it, renaming it and re-attaching it to the solution.